Invertebrate Gallery of the Indian Museum. 57 



tion. Some of them live as parasites on fishes, and one 

 of these parasitic forms is exhibited, with the front end of 

 its body embedded in the tissues of its host, — a fish of 

 the genus Macrurus. It must be remembered, however, 

 that this species, being a parasite, has undergone great 

 degeneration and has retrograded greatly from the typi- 

 cal Copepod form. A more typical form is Cyclops shown 

 in Case 48. 



iv. CRUSTACEA ENTOMOSTRACA CIRRIPEDIA. 



The Cirrtpedia are well known to sea-farers as Acorn- 

 shells and Barnacles, and in external appearance are quite 

 unlike Crustaceans, being commonly enclosed in a multi- 

 valve calcareous shell, somewhat resembling that of a 

 Mussel, which is fixed to foreign objects, often by means 

 of a long fleshy stalk. It is by following out their de- 

 velopmental changes that their Crustacean affinities are 

 clearly proved, for the young, or larval, Cirriped, far 

 from being fixed, is freely locomotive, and in all important 

 particulars resembles the early larval form of the Prawn : 

 it has eyes, antennae, and swimming-feet, but with increas- 

 ing growth it fixes itself by means of its antennae and by 

 a substance secreted by certain glands, known as cement- 

 glands, which open at the base of the antennae : after 

 attachment it loses its eyes, antennal filaments, and 

 swimming.feet, and developes forked cirriform feet and 

 an external shell. The adult Cirriped is thus immovably 

 fixed, head and mouth downwards, with its forked cirri- 

 form feet protruding between the valves of the shell 

 behind, the feet being used to sweep currents of water 

 containing food and oxygen towards the mouth. 



The internal structure of a typical Cirriped can be seen 

 from the dissections of Scalpellum in Case 48. 



A few of the Cirripedia are parasitic. One of these 

 parasitic forms is shown in the specimen of Sacculina 

 parasitic on the abdomen of a crab. Sacculina, as a re- 

 sult of its parasitic habits, has retrograded to such an 



