DEVELOPMENT OF PARAVORTEX GEMELLIPARA 459 



It is therefore obvious, since it agrees in all four of these 

 characteristics, that Linton's species must be referred to the 

 genus Paravortex. It now remains to determine wherein, if 

 at all, the American species differs from the two found in 

 Europe. Of Paravortex cardii Hallez gave a detailed description 

 which, translated, is as follows : 



''Vorticid provided with two ovaries, two reticulated and anas- 

 tomosing vitellogenous glands and two spherical testes; bursa 

 seminalis lacking. Genital pore ventral, situated close behind 

 the pharynx, at the end of the first quarter of the bod}^ Male 

 copulatory organ muscular, without chitinous parts and bear- 

 ing two lobes furnished with papillae. Pharynx doliiform. 

 Mouth ventral, near the anterior end of the body. Body cylin- 

 drical, thinner at the anterior end, uniformly ciliated, white 

 and with a slight yellowish tinge, transparent, without rhabdites, 

 very contractile, ordinarily bent in an arc and turning about 

 in one spot so as to describe a circle. Two black reniform eyes 

 above the pharynx. Maximum length, 1 mm.; diameter 0.3- 

 0,4 mm. Viviparous. Numerous soft-shelled capsules (up 

 to forty) each enclosing one to four embryos and distributed 

 through the connective tissue. Empty and rolled-up shells 

 remaining in the body of the mother. Lives in the stomach 

 of Cardium edule." 



Linton's species and P. cardii are closely similar both in struc- 

 ture and habits. Both have essentially the same color and 

 the same shape of body. Both have similar digestive, sensory 

 and glandular organs; both give birth to living young which 

 develop in capsules within the mother's body; both show the 

 same peculiar movements when taken from their host and 

 placed in sea water. 



The two species differ, however, in that the American form 

 attains twice the size of P. cardii; the genital pore is situated 

 farther posteriorly and the ovaries are longer in the latter; 

 an atrial canal in P. cardii leads from the dorsal part of the 

 atrium backward to the antrum femininum, while in the Amer- 

 ican species there is no distinct canal but rather the antrum 

 femininum extends backward from the middle of the posterior 



