Complex sentence: The animal is pale grey with darker head and tentacles but large in comparison with the shell and cannot completely retreat into it.The shell is subglobose, somewhat smooth, pellucid and greenish hyaline in colour.
Split into simpler sentences: The animal is pale grey with darker head and tentacles. It is large in comparison with the shell and cannot completely retreat into it.The shell is subglobose, somewhat smooth, pellucid and greenish hyaline in colour.