THE NEMATODE FAMILY GNATHOSTOMID^. 



275 



are found occupying cysts in the tissues of such bivalves as the 

 pearl-oystei' (see p. 277), where they appear to select chiefly the 

 adductor muscle of the shell as their habitat. 



Generic Diagnosis. 



EciiiNOCEPHALUS * Molin. 



Gnathostomin?e : head-bulb armed with transverse rows of 

 hooks ; not externally divided into swellings, but containing 

 four ballonets internally. Body unarmed. No cuticular collar 

 behind the head-bulb. Tail of male with slight alae and eight 

 pairs of papilhe, the most anterior pair always separated by a 

 long interval from the rest. Spicules slightly unequal (left 

 longer than right), tubular, long and slender, marked Avith 

 transverse striations. Yulva near posterior end of body. Yagina 

 long, opening into a wide uterine sac, which gives off two branches 

 anteriorly. Eggs oval, with thin shells ornamented with fine 

 granulations. Embryos not fully-formed at the time of laying. 



Habitat (adult) : intestine (usually in spiral valve region) of 

 sting-rays and other Elasmobranch fishes. 



Genotype: E. unchiatus Molin, emend. Baylis and Lane. 



Key to Species of Eclunoce'phalus 

 (excluding E. strlatus Mont.). 



A. Head-bulb witb not more tkau six rows of hooks E. nncinatus. 



B. Head-bulb with more than six rows of hooks. 



a. Dorsal and ventral lobes of lips each with two 

 teetb. 



a'. Head-bulb with 15 to 18 rows of about 150 to 



200 books each B. south ivelU {\). 



h'. Head-bulb with 30 to 40 rows-of xevy numerous 



hooks E. sjjinosissimus (p. 277). 



h. Dorsal and ventral lobes of lips each with a 



number (eight or more) of teeth E. nuiltidentatus {\).28o). 



1. EcHixocEPHALUs UNCINATUS t Molin, 1858. (Text-fig. 24.) 



Echinocej^halus uncinatus (in part) Molin (1858, p. 154). 



,, ,, (in part) Molin (1861, p. oil ; pi. xiii. 



figs. 7, 8)._ 



Cheiracanthus uncinatus v. Ijinstow, in Shiplev and Hornell 



(1904, p. 100 ; pi. iii. figs. 41, 44, 45-48). 

 Echinoc&phalus gracilis Stossich. in Shipley and Hornell (1906, 



p. 89). 



The original description of this, the type-species of the genus, 

 appears to us to have been based on two distinct species, both of 

 which we have been able to recognize among our material. The 

 head-bulb is described by Molin as having about 30 rows of small 

 hooks in the male, and only six rows of larger hooks in the 



* For measurements, see Table V., p. 288. 

 f For specific diagnosis, see p. 277. " 



Proc. Zool. Soc— 1920, No. XIX. 



19 



