CRANGON VULGARIS. 33 



oftliirteenannelidan Segments, is alone sufficieut to prove 

 this. The 11, too, we kiiow that this aiicestral raany-seg- 

 mentecl form must have crossed the liiie betvveen the au- 

 nelids and the arthropods, because nowhere among the 

 aiinelids do we find any form which can in any way be 

 compared to the Nauplius. That the archaic crustacean 

 possessed many ancestral features which do not appear in 

 the Nauplius can be seen from a study of the appendages 

 of Apus, which as Lankester ('81) has shown (rather than 

 that di- or trichotonious condition whicli is so often assigned 

 that Position) must be regarded as the primitive and typi- 

 cal crustacean appendage. Novv, a comparison of the tho- 

 racic appendages of Apus with the parapodia of some of 

 the more generalized worms belonging to the Polychaeta 

 (Errantia) shows clearly that it is from the latter that the 

 crustacean foot has been derived. 



Now it is reasonable to suppose that no matter what the 

 effects be on the young and on the race, it is an economy 

 to tlie parent in an oviparous form to send the egg out with 

 as small an amount of food yolk as possible, and then the 

 young finds it to its great advantage to escape from tlie egg 

 at as early a date as possible, provided it be equipped with 

 the necessary organs for playing its part in the world and 

 depending upon the efficiency of these for obtaining food 

 sufficient not only for its immediate wants, but for repro- 



of genitalia iu Craugon, Sedgwick ('88) shows that in Peripatus the 

 dorsal portiou of the coelom becomes restricted to the generative area, 

 so that if the ovary and testis of the Crustacea be horaologous with 

 those of Peripatas, the relations to coelom remaining the same, the 

 resemblances between the genital and segmental ducts will be even 

 more striking. The modiflcations of the primitive nephridial tubes of 

 the vertebrates into generative outlets will also suggest themselves in 

 this connection and their pertinence to the present discussion will be 

 more obvious when we remember that in many respects the verte- 

 brates are more vermian than are the Arthropods. 



ESSEX INST. BULLETIN, VOL. XXI 3 



