ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 623 



which are found in the hinder segment. The parent is tubular in 

 form, slightly enlarged at one end, completely smooth, with a delicate 

 cortical layer. Transitional forms between these two stages were 

 observed, and it was found that the anterior tuft of cilia on the embryo 

 disappears very early. The author thinks himself justified in regarding 

 this form as a distinct generic type. 



As has been pointed out by Huxley, we cannot form any definite 

 opinion as to the affinities of these forms until we know something 

 more about the characters of the infusoriform embryo ; Van Beneden 

 believed that this form might be the means by which the parasite passed 

 from one to another individual Cephalopod, but the researches of 

 Metschnikof and Julin on the history of the Orthonectida have largely 

 destroyed that hypothesis, and now give rise to the question whether 

 the infusoriform type is not the mature male, and its " urne " a testicle ; 

 to see if this view could be confirmed the author closely examined 

 them in the hope of detecting spermatozoa, but in vain. On the other 

 hand, we now know that the spermatozoa of the Orthonectida are very 

 small and very difficult to see. Too much weight must not, therefore, 

 be given to this negative result. And the hypothesis which suggested 

 the investigation commends itself, on many considerations, to the 

 author's mind. 



When the first studies on the Dicyernidae were published, the 

 absence of any mesoderm was insisted on ; since that time, and chiefly, 

 thanks to the researches of the brothers Hertwig, it has been allowed 

 that the mesodermal formations in the Metazoa have not all the same 

 anatomical value. Still, it remains true that the Dicyemidaa have 

 neither mesenchyma nor ccelomic folds. This being so, we find in 

 this important fact a justification for the establishment of a phylum 

 of Mesozoa. These may be defined as " organisms formed of two 

 layers, the ectoderm of a layer of more or less ciliated cells, the endo- 

 derm of a single or of several cells. The sexual products arise from 

 the endoderm. There is no mesenchyma, ccelomic folds, or any funda- 

 mental lamella ; two female forms, one arising exclusively from the 

 males, the other from the females. All the Mesozoa actually known 

 are parasites." They are divisible into the Orthonectida and the Khom- 

 bozoa ; the former have the body composed of several rings and the 

 endoderm of several cells, some of which take on the epithelio-muscul'ar 

 type, and give rise to muscular fibrils, while the others form the sexual 

 products. The male is elongated, and the female oviparous. The 

 Bhombozoa, which are divisible into the Dicyemida and Heterocyemida, 

 have the body annelated, the endoderm formed of a single cell, no 

 muscular fibrils. The male is peg-top in shape, refractive bodies are 

 found in some of the smooth anterior cells, and the females are 

 viviparous. 



The author discusses the question as to whether the Mesozoa are 

 Metazoa degenerated by parasitism, and specially the doctrine of 

 Leuckart,* that they are comparable to the ciliated larvae of the 

 Distomata. He points out that the resemblance is chiefly external, 

 and that a Dicyema is much more like a Planula or a Gastrula, and 



* See tlii8 Journal, ante p. 313. 



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