On _Echinodermata_.--(_a_) _Crinoidea._ _Stylina comatulicola_ lives on _Comatula mediterranea_, fixed to the outer skin, which it penetrates by a very long proboscis; the shell is quite transparent.[164] A curious case of a fossil parasite has been noticed by Roberts.[165] A _Calyptrea_-shaped shell named _Platyceras_ always occurred on the ventral side of a crinoid, encompassed by the arms. For some time this was thought to afford conclusive proof of the rapacity and carnivorous habits of the echinoderm, which had died in the act of seizing its prey. Subsequent investigations, however, showed that in all the cases noticed (about 150) the _Platyceras_ covered the anal opening of the crinoid in such a way that the mouth of the mollusc must have been directly over the orifice of the anus. (_b_) _Asteroidea._ The comparatively soft texture of the skin of the starfishes renders them a favourite home of various parasites. The brothers Sarasin noticed[166] a species of _Stilifer_ encysted on the rays of _Linckia multiformis_. Each shell was enveloped up to the apex, which just projected from a hole at the top of the cyst. The proboscis was long, and at its base was a kind of false mantle, which appeared to possess a pumping action. On the under side of the rays of the same starfish occurred a capuliform mollusc (_Thyca ectoconcha_), furnished with a muscular plate, whose cuticular surface was indented in such a way as to grip the skin of the _Linckia_. This plate was furnished with a hole, through which the pharynx projected into the texture of the starfish, acting as a proboscis and apparently furnished with a kind of pumping or sucking action. Adams and Reeve[167] describe _Pileopsis astericola_ as living ‘on the tubercle of a starfish,’ and _Stilifer astericola_, from the coast of Borneo, as ‘living in the body of a starfish.’ In the British Museum there is a specimen of _Pileopsis crystallina_ ‘in situ’ on the ray of a starfish, (_c_) On the brittle starfishes (_Ophiuroidea_) occur several species of _Stiliferina_. (_d_) _Echinoidea._ Various species of _Stilifer_ occur on the ventral spines of echinoids, where they probably subsist on the excreta, and are sometimes found imbedded in the spines themselves. _St. Turtoni_ occurs on the British coasts on several species of _Echinus_, and _Montacuta substriata_ frequents _Spatangus purpureus_ and certain species of _Echinocardium_, _Cidaris_, and _Brissus_. _Lepton parasiticum_ has been described from Kerguelen I. on a _Hemiaster_, and a new genus, _Robillardia_, has recently been established[168] for a _Hyalinia_-shaped shell, parasitic on an _Echinus_ from Mauritius. (_e_) _Holothurioidea._ The ‘sea-cucumbers’ afford lodgment to a variety of curious forms, some of which have experienced such modifications that their generic position is by no means established. _Entoconcha_ occurs fixed by its buccal end to the blood-vessels of certain _Synapta_ in the Mediterranean and the Philippines.