THE COPEPODA , 93 



first Copepodid stage, and the later stages of the typical series are 

 suppressed, the characteristic features of the parasite being assumed 

 at the next moult. 



In the Choniostomatidae, which are parasitic on other Crustacea, 

 the larva hatches in the first Copepodid stage, with two pairs of 

 swimming-feet, and becomes attached to the host by an adhesive 

 frontal plate, corresponding probably to the "frontal thread" of other 

 forms. This larva may give rise at once to the adult form, or a 

 pupal stage with reduced limbs may intervene. The male is similar 

 to the female in essential structure, but of much smaller size. 



The most extreme stage of degeneration is reached in the 

 Herpyllobiidae, which are parasitic on Polychaete worms and on 

 Crustacea. In Rhizorhina, which is the most thoroughly known, 

 the adult female is entirely without appendages, and is attached by 

 a tubular process which ramifies within the body of the host in a 

 fashion recalling the " roots " of the Rhizocephala. The adult male 

 is also entirely limbless, but remains enclosed within the last larval 

 skin (of the first Copepodid stage), ^^ r ithin it are formed a pair of 

 relatively enormous spermatophores, which are not expelled from 

 the body but discharge their contents through ducts which pass 

 out close to the point of attachment, and in front of the position 

 of the larval mouth. A number of males are attached to the body 

 of the female near the genital apertures. 



The Monstrillidae have a very remarkable life-history, which 

 has only recently been made known. The adults of both sexes are 

 free-swimming, and are without mouth-parts or alimentary canal. 

 The newly hatched young are also free-swimming, but the inter- 

 mediate stages are endoparasitic within various Polychaete annelids. 

 The life-history is most fully known in the case of Hatmocera danae, 

 investigated by Malaquin (Fig. 53). The adults (F) have antennules 

 and four pairs of swimming -feet, but no trace of antennae or 

 mouth-parts. The fifth pair of thoracic feet are vestigial in the 

 female and are stated to be absent in the male. The alimentary 

 system is represented by a blind stomodaeal invagination and a 

 mass of undifferentiated endoderm cells. The female carries a 

 single packet of eggs (ov) adhering to a pair of very long setae 

 (g.s) which spring from the genital valves. The young are hatched 

 as nauplii, without mouth or alimentary canal, and with strong 

 hook-like mandibles (A). The nauplius burrows into the body of 

 its host (the Polychaete Salmacina), casting its cuticle in the 

 process and losing its limbs, so that when it reaches the body- 

 cavity it consists merely of a mass of embryonic cells without a 

 cuticle (B). From the body-cavity the parasite passes into the 

 vascular system of the host, where it undergoes its further develop- 

 ment. A thin cuticle is now secreted, and a pair of processes begin 

 to grow out from one end of the ovoid body (C, pi'). These 



