348 CKESSWKLL SHEAHEIJ. 



degree of development, and for this reason can hardly he 

 compared witli that of Dinophilus; in hoth, however, the 

 main musculature consists in a series of longitudinal ventro- 

 lateral and dorso-lateral muscles. The alimentary canal shows 

 the same divisions, although differing considerably in the 

 relative proportion of its parts. The sti-ong chitinous jaws 

 are wanting in Dinophilus. In each the cavity surround- 

 ing the gut is a primitive blastocele with no definite epithelial 

 lining. This cavity sends prolongations into the head. The 

 equivalent of the coolom in both is repi-esented by the cavity 

 of the reproductive glands. In the male these consist of a 

 more oi- less paired testis, vesicuhe, vas deferens, and 

 median penis, and in the female a large ovarian cavity, 

 paired or nnpaired, with oviducts. 



With Annelids Dinophilus shows a closer relationship 

 than Histriobdella, mainly due to its less direct develop- 

 ment. In fact the development of Dinophilus brings it 

 into line with that large gi'oup of animals such as thePoly- 

 chgetas, Echiuridse, Gephyrea, Laraellibranchs, and 

 the Gasteropoda, in having the ectoderm arising from the 

 first three quartettes, mesoderm from the left posterior cell of 

 the fourth quartette (4 d.), and the endoderm from the remain- 

 ing cells. In the derivation of a large part of the ectoderm 

 of the trunk from the posterior cell of the second quartette 

 the resemblance to the Pol3'cha3t Annelids is most pronounced. 

 This is further enforced in the origin of the bilateral cleavages 

 in the cross cells and in the products of 2 D. "The transition 

 from the spiral type of cleavage to the more specialised 

 bilateral type occurs in precisely the same directions as in 

 the Polychasts. Moreover the second bilateral divisions of 

 the cells of the posterior arms of the cross continue this 

 resemblance. All these characters, if such thev mav be 

 called, when viewed as a whole point in no uncertain way 

 to the descent from the Annelid stem, and at a point not far 

 fi'om that at which the Polychasta arose" (Nelson, p. 728). 



The weio-ht of our evidence, furnished bv recent work on 

 the morphology and embryology of Dinophilus, is strongly 



