The Morphology of the Nudibranchiate Mollusc Melibe (syn. Chioraera) leonina (Gould)
Authors: Agersborg H. P. Kjerschow 
Published: April 2021

ABSTRACT
It is evidenced by the work of various authors that Gould’s Chioraera (1852) is identical with Bang’s Melibe (1829); Chioraera, therefore, is a synonym of Melibe; Gould, at the time of his description, did not know of the genus discovered by Rang. For these reasons I have consistently named it throughout all my works on this species Melibe leonina (Gould) in spite of the attempt of certain authors to build on the nomenclature of Gould. Melibe leonina is absolutely void of masticatory organs ; the generic description of Gould may be augmented, therefore, to read in part, Bulbus pharyngeus aut cum mandi-bulis aut sine mandibulis ; radula et lingua destitutus. The anterior end of M. leonina is formed into a large cowl ; this has a pair of stalked foliaceous tentacles which may be retracted below the edge of the stalk which then acts as a sheath. The tentacles are very complex in structure, being innervated with nervous tissue. The tentacles are not ciliated, as claimed by Jeffreys (1869) for all nudibranchs. The cowl is fringed with two rows of cirrhi which also are highly complex in structure. From an inner ganglionic axis, nerve-fibres radiate to the peripheral ectoderm of the cirrhus. The exact function of the tentacles, as well as of the cirrhi, is not known. The tentacles are commonly called rhinophoria but, since the exact function is not known, I have employed the original term tentacles (dorsal tentacles) instead of the commonly used term ‘rhinophoria ‘. The cirrhi are more sensitive to tactile stimulus than are the dorsal tentacles (Agersborg, 1922 a: 441-3). The body-surface of M. leonina appears smooth, but upon close examination it is found to be everywhere tuberculate, including the sole of the foot and the ventral side of the hood ; in that way this species corresponds to other members of this genus. The dorsal appendages, which in M. leonina consist of six pairs of foliaceous lobate structures, I have called by the Linnean term papillae, instead of cerata, or branchial papillae for the reason as stated by Bergh (1879 c) : ‘Respiration takes place all over the surface in Nudibranchs,’ &c. The papillae alternate in position ; they are subject to variation in structure relative to position and age (Pl. 27, figs. 1, 2, 3, and Pl. 30, figs. 18–25). The foot projects in front of, and behind, the main body. It is highly tuberculate and ciliated. Internally, a fine nerve network is seen spread throughout its length and breadth, and at the posterior end it aggregates into a ganglionic centre. Fine nerve fibrils are seen to pass to the ciliated ectoderm. A great many mucous glands are present all through the foot, which open independently through small crypts between the ectoderm cells. These glands are the pedal glands which are scattered all through the foot (figs. 1, 2, 3, 9, 26, 27, 28, 29). There are three kinds of glands in the body-wall : (1) the largest and most numerous are the odoriferous glands ; (2) the next in size and number are the saccular mucous glands ; and (3) the unicellular mucous glands (Pl. 31, figs. 30, 31). The muscle system lies below the glandular fimbriated ectoderm ; the muscle-fibres are arranged in a fashion like the fibres in a basket (Pl. 27, fig. 3). The muscle-cell consists of two sarcoplasmic regions each containing an abundance of micromeres : those in the inner region are larger (Pl. 31, fig. 33, C’a) than those in the outer region. The larger are called in the text macromeres. Each of these micro-macromeric sarcoplasmic regions is invested by a coarsely granular net-work (My, Sar). The nucleus is placed centrally within the cell. Its chromomeres (Ik) are scattered differently, i. e. sometimes around the periphery of the nucleus (Pl. 31, figs. 32, 33, Pl. 37, fig. 82) and sometimes less so. There is no definite body-cavity. The body-cavity as it exists corresponds to the primary body-cavity of Lang (1896) or the perivisceral cavity of Sedgwick (1898). The alimentary canal consists of five regions : The oesophagus with the non-glandular lining, and back of it the oesophageal glands or salivary glands (Pl. 32, figs. 36, Oe, 41, Sg). The proventriculus with distinct glandular lining (Pl. 32, figs. 36, G, 38, GV). The gizzard with its stomach-plates which are formed by the secretion of the epithelial lining (vide supra), (Pl. 32, figs. 37, 42, 43, StpV). The pyloric diverticulum with its glandular and ciliated and much corrugated surface, which secretion does not keratinize as that of the gizzard (Pl. 32, figs. 35, 39, 44). The intestine with its large typhlosole protruding into the cavity from the ventral side, and glandular ciliated surface (Pls. 32 and 33, figs. 40, 45, 46, 47). The structure of the intestinal lining is unique as concerns the fibrillar nature of the cytoplasm, the clearly visible terminal bars, and the basement membrane ; also, the non-convergence of the cytoplasmic portion of the cilia on the nucleus. The liver ramifies all the parts of the body. Its secretion into the gizzard does not harden in the alimentary canal. The glandular structure of the epithelium of the liver exhibits that it has an active function in vivo, owing to the presence of a variable series of granules and vacuoles in the adjacent cells fixed at the same time in the same way (Pl. 27, figs. 1, 2, 4, 7, Pl. 33, 48-53). The heart consists of two chambers enclosed within the pericardium. These chambers are separated by valves from the efferent branchial veins and the afferent aortic trunkvessel. In the aorta, just below the valve of the ventricle, is a blood-gland or node which contains pseudopodic cells. The cells found on the outside of this node, i.e. within the lumen of the aorta, are different in structure from those found within the node (Pl. 34, figs. 54-9). The wall of the heart consists of epithelioid and some semi-musculofibrilloid cells (Pl. 34, fig. 59). The kidney is much branched, and is situated between the pericardium and the gonadium. It communicates with the pericardium through the renal syrinx which is closed at the point of junction with the pericardium by what I have called a cyncitial plate. The lining of the kidney is glandular ; so is also that of the ureter, but neither is ciliated. The renal syrinx, however, is ciliated. The cells of the renal syrinx are peculiar in that the cilia are very large and independent in position, i. e. they do not mingle with those of adjacent cells. The renal syrinx is plicated, and from the cyncitial plate a ciliated villus protrudes into the organ (Pls. 35 and 36, figs. 64, 65, 67, PT). The organs of reproduction represent an additional type to the three types enumerated and described by Lang (1896). The male and female ducts in M. leonina do not unite to form a common atrium genitale as set forth by Lang for all nudibranchs and a few tectibranchs, but open close together through separate apertures (Pl. 37, fig. 77, Mgp) ; the penis lies in front of the vagina (Pl. 28, fig. 9, P) ; in that way it resembles the second type of Lang. Both genital ducts pass independently to the same acinus ; in this respect it differs from all three types of Lang, and, for this reason, I have designated the genital duct system in this mollusc as constituting a Fourth Type. During any ripe phase of the gonads the duct leading from the inactive area of an acinus may be quite obscured by the ripening mass of germ-cells. The organs of reproduction are represented in Pl. 37, fig. 81. The spermatotheca I have called ovispermatotheca because it is frequently filled with eggs from the oviduct. The structure of the ovispermatotheca is quite peculiar owing to the plicated nature of its lining. The cells lining this organ are flask-shaped, the neck being longer than the body and abutting into the cavity (Pl. 37, fig. 82). The mucous gland is relatively rather large. A great deal of mucus is formed by this gland at the time of oviposition (Agersborg, 1919, 1921, 1923 a). Sections of the gland which I have studied show that during the act of mucus-formation the nucleus takes an active part. The nuclear membrane is then very obscure or apparently absent, the nucleus goes through fragmentation, the smallest cytoplasmic granules of basophil nature are the nearest to the nucleus ; after the mucous granules have liquefied and passed into the lumen or cavity of the gland, the nucleus of the gland-cells is small, shrunken, and non-granular, with a small nucleolus, and basal in position within the cell. The cell then passes through a period of refilling during which time the nucleus first grows in size, and at the same time the micromeres of the cytoplasm, which are lodged in the meshes of the reticular structure of the cell, also grow.