386 BULLETIN" 15 8, UNITED STATES NATIONAL MUSEUM 



males. Urosome cylindrical and considerably narrower than the 

 metasome; eggs usually received into an incubatory pouch and not 

 carried in external ovisacs. First antennae short, made up of few 

 segments, both of them prehensile in the male; second antennae 

 without an exopod and distinctly prehensile, with one or more apical 

 claws. First four pairs of legs usually biramose, sometimes unira- 

 mose, one or more pairs often lacking; fifth legs uniramose, 1- or 2- 

 segmented. Living either as commensals within the body of ascid- 

 ians and other invertebrates, or as parasites upon their outer surface. 



Remctfrhs. — Every genus belonging to this group is either para- 

 sitic or commensal, and at least 80 per cent of them live within 

 ascidians. As a result of this mode of life most of them are pro- 

 foundly modified in body form and in the structure of the various 

 appendages. This modification, however, has not been carried far 

 enough to destroy the systematic value of the appendages, and they 

 still afford the best means of identification. But it has resulted in 

 a marked sexual dimorphism; the great majority of the males retain 

 throughout life the ability to swim about freely, while the females 

 on reaching maturity become confined to the body of their host. 

 The male accordingly retains most of the characteristics of free 

 swimmers, and the female loses them to a greater or less degree. 

 Consequently the sexes must be separated in the key (Appendix B, 

 p. 598) with but few exceptions, and in more than half of the genera 

 the male unfortunately still remains unlaiown. 



During the years from 1864 to 1880 Hesse established many new 

 genera among these ascidian commensals, but both his descriptions 

 and the accompanying figures were so inaccurate and contradictory 

 as to possess almost no systematic value. Many of his genera be- 

 come mere historical curiosities, as Chatton and Brement have well 

 pointed out, but the fact still remains that if any of them become 

 capable of recognition, as has already happened in many instances, 

 they must retain the names he gave them. For this reason alone, 

 and not because they are regarded as being at present valid, they are 

 located exactly according to the characters he gave them. 



Family NOTODELPHYIDAE 



Genus NOTODELPHYS Allman, 1847 



Head fused with first segment, second and third segments free and 

 separated by deep lateral sinuses; fourth and fifth segments fused, 

 dilated laterally and strongly arched dorsally to form brood pouch. 

 Urosome narrow cylindrical, 5-segmented; genital segment not en- 

 larged in female but swollen in male ; caudal rami much longer than 

 wide, ciliated on both lateral margins. First antennae 15-segmented ; 



