596 THE BIOLOGY OF MARINE ANIMALS 



There are several species of eunicids which are parasitic during their 

 young stages in various other polychaetes and echiuroids, for example, 

 Oligognathus bonelliae in Bonellia, Drilonereis in Protula and Labrorostra- 

 tus parasiticus in Odontosyllis (75). The parasites show no regression apart 

 from a slight simplification of jaws. They invade the host when the latter 

 is still very small, increase in size with it and finally leave it to reproduce. 

 Other polychaetes are also parasitic in their early stages. Thus the alciopid 

 larva of Corynocephalus albomaculatus is found in the digestive cavity of 

 the ctenophore Cydippe. 



More highly specialized are the Histriobdellidae, of which Histriobdella 

 homari is found in the gill chamber or among the eggs of the lobster. These 

 animals are simple in structure and are sometimes included among the 

 archiannelids. The myzostomarians are well-known associates of feather 

 stars, crinoids and ophiuroids. Some species, e.g. Myzostoma cirriferum 

 on Antedon, are commensals rather than parasites. Others, more sedentary, 

 form lesions in the skin of the host, whereas true endoparasitic species like 

 M. pulvinar, which is found in the crinoid Septometra, invade the digestive 

 tract of their hosts (3). 



The Hirudinea contain some marine species, for example Pontobdella 

 and Branchellion, parasitic on rays, skates and sharks. These leeches are 

 blood-suckers with biting mouth parts, and attach themselves by means of 

 suckers to fish they encounter. 



Molluscs. Molluscs, like annelids, constitute a group in which parasitism 

 is the exception rather than the rule. One family of lamellibranchs, the 

 Lucinidae, contains members more or less parasitic on echinoderms. 

 Among gastropods there is a variety of ecto- and endoparasites showing 

 different degrees of specialization and regression of organs as the result of 

 parasitism. The forms associated with echinoderms, in particular, afford a 

 remarkable series showing alterations of structure along the road of a 

 parasitic existence (Fig. 14.10). 



Certain species of Eulima (Eulimidae) are still free-living, or exist in 

 commensalism with echinoderms. Mucronalia variabilis (Stiliferidae) is 

 another gastropod living on the skin or in the digestive tube of 

 holothurians (Synapta). Mucronalia pierces the wall of the sea cucumber 

 with a long proboscis and sucks in the coelomic fluid of its host. It is little 

 modified externally, but the digestive system shows profound changes : the 

 proboscis is enlarged, the radula is absent and the oesophagus terminates 

 as a short blind tube. Stilifer linckiae, an ectoparasite of the sea star 

 Linckia is interesting in showing great development of a pseudopallium, 

 which grows back to envelop the entire shell. This is a new structure, 

 which takes the form of a collar surrounding the mouth region. There is 

 some regression and modification in the gut : the proboscis is enlarged, the 

 radula missing and the liver reduced. Stilifer pushes its proboscis into the 

 coelom of its host and feeds on tissue fluids. 



The genus Thyca (Capulidae) displays slight modifications towards an 

 ectoparisitic condition involving the foot and peribuccal region. The former 



