ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 903 



the invagination for the proboscis ; the anterior thickened parts form 

 the rudiments of the ventral and dorsal lobes of the brain, and the 

 posterior those of the lateral nerve-trunks. The ventral commissure 

 of the central ganglia is formed by the fusion of the processes of the 

 two ventral lobes, and appears much earlier than the dorsal commissure. 

 The lateral nerves are formed as direct continuations of the primitive 

 nervous rudiments, are at first confined to the cephalic region, and 

 only later make their way into tbe trunk. The author concludes 

 from these results that the cerebral ganglia of Nemertines are 

 homologous with those of Annelids ; that the ventral commissure of 

 the former corresponds to that which lies between the two halves of 

 the Annelid brain ; that the dorsal commissure is peculiar to Nemer- 

 tines, and that their lateral nerves correspond to the oesophageal 

 commissure of Annelids. Prof. Salensky discusses the recent studies 

 of Prof. Hubrecht, who asserts the mesodermal origin of the nervous 

 system of Nemertines, and expresses doubts as to the correctness of 

 the observations. 



The sheath of the proboscis first appears as a mass of mesodermal 

 cells placed at the tip of the proboscis-invagination ; as the proboscis 

 and its sheath grow backwards, the epithelial layer of the former 

 appears as an elongated closed sac with a dorsal direction ; the meso- 

 dermal portion of the proboscis — the muscular layer and the sheath — 

 have the form of a double-walled cap which surrounds the greater 

 part of the epithelial layer. The mode of development of the pro- 

 boscis of Pilidium is completely similar to that of Monopora ; here 

 again the author's results are in opposition to those of Prof. 

 Hubrecht, while they confirm his own earlier proposition that the 

 Nemertine proboscis is to be derived from that of the Turbellaria. 



Modification of the Trochal Disc of the Rotifera.*— Prof. A. G. 

 Bourne writes :• — It is now a generally accepted theory that this 

 structure is the homologue of the ciliated bands of the larvae of Echi- 

 noderms, Chsetopods, Molluscs, &c, and of the tentaculiferous 

 apparatus of Polyzoa and Gephyrea, and is often termed in common 

 with these a " velum." This velum presents itself in various stages 

 of complexity. It is found as a single circum-oral ring (Pilidium), 

 as a single pras-oral ring (Chaetopod larvae), or as a single prae-oral 

 ring, coexisting with one or more post-oral rings (Chaetopod larvae, 

 Holothurian laivae). We may here assume that the ancestral con- 

 dition was a single circum-oral ring associated with a terminal mouth 

 and the absence of an anus, and that the existence of other rings 

 posterior to this is an expression of metameric segmentation, i. e. a 

 repetition of similar parts. With a development of a prostomiate 

 condition, a certain change necessarily takes place in the position 

 of this band ; a portion of it comes to lie longitudinally, but it may 

 still remain a single band, as in the larvae of many Echinoderms. 

 How have the other above-mentioned conditions of the velum come 

 about ? How has the prae-oral band been developed ? Two views 

 have been held in regard to this question. 



* Rep. 55th Meeting (1885) Brit. Assoc. Adv. Sci., 1886, pp. 1095-6. 

 Ser. 2.— Vol. VI. 3 T 



