# Natural History Review/Series 2/Volume 2/Number 5/Observations on some Australian and Feegeean Heterocyathi and their Parasitical Sipunculus

Is two separate casts of the lead off the Bellona Beef, Lat. 21. 51. S., Long. 159. 28. E., we obtained specimens of living Polypi, referable, as Dr. Gray has since very kindly informed me, to the genus Heterocyathus, and on comparing them with others previously collected by me in the Feegee group, I found that they were specifically different, though obviously belonging to the same genus.
The corallum is simple, free, depressed, broad and flattened at the base, becoming smaller towards the calyx or oval disc, which is more or less oval in figure, and comparatively shallow, with a well-developed septal system following the regnant number six.
The septa are disposed in three sets, or whorls, according to the order of their development, viz. a primary set, which is most prominent and made up of six or twelve members, a secondary, equal in number and alternating with these, and a tertiary set, of double that number and alternating with the other two. The primary septa have, on either side, a thin sub-parallel lamina, with which they are blended at the thecal margin, being only connected with them internally by means ​of the columella. The secondary septa are furnished with laminæ of the same description, which join those of the first set, at an acute angle, without reaching the columella; and the tertiary septa pass into this point of union, having no supplementary laminæ of their own. The two sets of plates, just noticed, present a rounded shoulder internally (more prominent in the primary ones) giving them the character of lateral pali, or dismemberments of the septa.
The columella is composed of a spongy tissue, with an oval and slightly convex summit.
All the plates of the disk are spongy, or minutely granular, on the surface, but compact within. The body of the corallum is spongy at the axis, in continuity with the columella, more compact below and around this, and again more porous towards the exterior, especially above.
The loculi are circumscribed, but not crossed by synapticulæ or interseptal dissepiments. They are just double the number of the septa, lying one on either side of the later, and are thus arranged by pairs in three distinct circles; the internal corresponding with the primary, and the external with the tertiary rays.
In the species taken at the Bellona Shoals the oral disk was distorted, with a central constriction, as though the process of fission had been going forward. In one specimen indeed the opposite margins of the disk had actually coalesced. The primary septa were twelve in number, and all the plates are so much compressed that the loculi are exceedingly narrow. The external surface of the corallum is beset with minute granulations disposed in broke latitudinal lines with porous channels between them; on the other hand, in the Feegean species the disk is regular, with six primary rays and wider loculi, and the external surface of the corallum, is coarsely granulated, without any very obvious linear disposition, as the first rudiments of costæ.
In a recent visit to Moreton Bay we dredged (in a few fathoms depth) two beautiful specimens of another species of this genus, differing from the foregoing in having well marked longitudinal costæ, exactly forty-eight in number, and corresponding, each for each, with all the radiating septa and laminæ, with which they are directly continuous at the margin of the disk. The principal laminæ are falcate towards the hollow of the cup and deeply notched, toothed and echinate, as they pass into the spongy coiumells, whose actual limit is thus rendered less definite than in the other species described.
Of the soft core parts of this polyps, I can say but little. They appear to be very scanty, from the fact, that when the animals are immediately taken from the water there is scarcely anything to be seen but a brown, soft and tenacious matter, filling up the crevices of the skeleton above described, and all the prominent points and ridges become quite bare. The whole surface of the corallum is covered over with a thin ectodermic layer, which however is much worn at the ​base. In the fragments removed very cautiously with a knife, I could recognise what appeared to be portions of simple tentacula, muscular fibres and a fibrous matrix, brown pigment cells, the usual yellow or amber-tinted spherical globules of the subjacent parts, and ciliated epithelium, from the lining of the common cavity. The analogy of other cases suggests the probability that the tentacula correspond with the outer whorl of loculi.
The most remarkable circumstance connected with these polyps, is the invariable presence of a little solitary Sipunculus in a beauti- fully excavated burrow at the base of the corallum. The uniform position of the opening and sinistral direction of this burrow, first observed in dead specimens, led me to suppose that it was in some way connected with the economy of the polyps themselves, but having discovered its occupant to be one of the coral perforating Sipunculidæ, which abound in the South Seas, the riddle was quickly solved. The body of one of these parasites, taken from a Bellona Beef specimen, is about 3/4 of an inch in length, terete, but gradually increasing in diameter from before backwards, and exhibiting a permanent curvature forwards, corresponding with that of the burrow.
The crested proboscis is about three times the length of the body, and crowned with simple ciliated tentacula. On the dorsal surface, immediately behind the base of the proboscis, is a little oval and brownish callosity, answering the purpose of an operculum, when the animal is retracted into its cell, and close behind this disk is the anal aperture. The posterior extremity of the body is furnished with a similarly constituted, but slightly conical, shield. As the opercular disk meets the rest of the dorsal surface, at an angle more or less obtuse, the proboscis appears to hold a subterminal ventral position, and protrudes itself somewhat perpendicularly to the axis of the body. The surface of the latter is beset with minute asperities, disposed serially, or irregularly scattered. These become larger and more numerous towards the dorsal region, and more definitely aggregated at the extremities; they constitute the before mentioned opercular and caudal disks. As they extend themselves on the proboscis they grow smaller, and begin to assume a more orderly arrangement, and finally form into closely set rings of minute and recurved hooks, reaching to the base of the oral tentacula. This parasite is evidently closely allied to the little animal from the Indian seas, named Lithodermis cuneus by Cuvier, and which was the only species known to him.
In the Coral borers, which are nearly identical with the little animals here noticed, I found that the œsophagus was encircled by a nervous collar, with a cephalic enlargement on either side, from which tentacular nerves arose, and, in contact with which, dark eye specks were distinctly visible; there is also a single ventral nervous chord, giving off lateral nerves at stated intervals, but without any very apparent ganglionic dilatations. I observed, moreover, that the cavity of ​the body was lined with a ciliated membrane, which was reflected round the larger branches of a transparent (probably water) vascular system running along the spirally coiled intestine, with its singularly constructed central suspensory ligament. All this militates against the supposed Echinoderm nature of Sipunculus, and give it radiating affinities with the Annelida, Polyzoa and Tunicata, though perhaps only of a representative kind. The simple anatomy of the larval form, the Atlas of Peron, if it be not indeed a permanent one, is also of great importance in this connexion.
References.
1. Sipunculus Heterocyathi, (natural size) a. Opercular disk. b. caudal disk.
2. Ventral view enlarged, a. Opercular disk, b. caudal disk, c. proboscis.
3. Dorsal view enlarged, a. Opercular disk, b. anus, c. proboscis.
4. Corallum, seen from above, enlarged, a. Columella, b. primary. c. secondary and d. tertiary rays. e. primary. f. secondary and g. tertiary loculi. h.h.h. Openings communicating with the burrow.
[The case of parasitism here described has an especial interest if viewed in relation to the apparently anomalous characters of the palæozoic Pleurodictyon, which we are much inclined to regard as a coral perforated in a similar manner. —Eds..]