ARTHROPODA . 277 



pedia are hermaphrodite, there are certain genera, e.g. Ibla and 

 Scalpellum, in which males have been described. These are 

 dwarfed forms, often very degenerate, which live usually two or 

 three at a time within the mantle of the normal hermaphro- 

 dite or female, and are known as complemental males. 

 Their degree of degeneracy varies within wide limits, and it 

 seems as a rule to increase with the depth of the water in 

 which they live. Three species of Scalpellum which inhabit 

 shallow water, S. peronii, S. rostratum, and S. villosum, are pro- 

 vided with males which have retained the division of the body 

 into a capitulum and a peduncle ; in eight other species, which 

 extend to a depth of 700 fathoms, the males have lost this 

 division of the body, but still retaia rudiments of shell 

 valves ; in thirteen other species, which were almost all taken 

 from depths of upwards of 1000 fathoms, even these have dis- 

 appeared. The male of Scalpellum regium, one of the most 

 degenerate, is a minute animal about 2 mm. long, attached to the 

 inner surface of the scutum of the female by means of minute 

 antennae which are provided with cement glands. No other 

 appendages are visible. The body is oval, rounded at each 

 end, and covered with a chitinous cuticle, which is produced 

 into short spines. The alimentary canal is rudimentary and 

 functionless, the mouth does not open, and the mouth append- 

 ages are gone. There are a supra-oesophageal ganglion, con- 

 necting cords, and one infra-oesophageal ganglion ; the eyes 

 have disappeared. The generative apparatus alone is well 

 developed, but the female organs have completely disappeared, 

 the testis and vesicula seminalis are single. 



The remarkable genus Scalpellum presents yet a further 

 complication in its sexual arrangements. Its species may be 

 arranged in three groups : 



A. Those which are truly hermaphrodite, and in. which no 



complemental males exist. Ex. Scalpellum halanoides. 



B. Those which are hermaphrodite and yet possess comple- 



mental males. Ex. Scalpellum villosum, peronii, vulgar e, 

 rostratum, etc. 



C. Those which are unisexual, the females large and normal, the 



males minute and living parasitically in the mantle of 

 the female. Ex. Scalpellum ornatum, regium, vitreum, etc. 



