Anatomy and Histology of Nemertines. 397 



displays a marked conformity with tliat of Groups T. ami II. 

 in that it likewise consists of circular and longitudinal layers 

 of fibres, in addition to whith, in the case of many Rhabdo- 

 ccela, we also have a diagonal layer, lying between the two 

 former. ^luch more complicated is the musculature of the 

 body-wall in the Polyclads, in which, according to Lang"^, as 

 many as six layers may be present, arranged in the following 

 order : — circular, longitudinal, diagonal, circular, diagonal, 

 longitudinal. In this case also it is at once evident that only 

 the internal layer of diagonal fibres has to disappear in order 

 that we may get the arrangement of the muscle-layers found 

 in Group II., and in Cerehratulus in particular. 



I have alluded to the fact that the integument, and 

 es])ecially the ectoderm, is composed of fibrillar and gland- 

 cells, exactly like the hypodermis of the Annelids, among 

 which I should like to see the Gephyreans included. It 

 remains to be added that the ectoderm of Nemcrtines is clothed 

 by a cuticle, which may be provided with cilia in places. As 

 a general rule a cutis is not present in the Annelids ; yet in 

 the case of Sipunculus nudus, for example, this has recently 

 been described by Andreaj f, who states that it contains 

 pigment-masses and gland-cells. The phenomena presented 

 by the hypodermis of tlie Annelids and the ectoderm of the 

 Nemcrtines at the time of sexual maturity are very remark- 

 able ; in both cases the naked gland-cells swell up to a large 

 size, almost entirely filling up the epidermis around the genital 

 apertures (clitellum of the Earthworms, porophore of the 

 Capitellida,') %. The musculature of the body-wall of the 

 Annelids is allied to that of Groups I. and ILL, since it con- 

 sists of a circular and a longitudinal layer. If we neglect 

 the fact that the diagonal layer, which is stated by Andreae 

 to lie in Sipunculus between these two muscle-layers, does 

 not entirely agree in structure with that of the Nemcrtines, 

 the musculature of the body-wall of a Carinella or a Drepa- 

 nopkorus would be essentially the same as that of the 

 Gephyrean. 



In all Nemcrtines the parenchyma is developed to its 

 utmost extent, and the organs are consequently imbedded in 

 a gelatinous tissue. In the case of Groups II. and III. this 

 tissue is arranged in septa in the region of the mid-gut, and 



* Lang, " Die Polycladen des Golfs von Xeapel " (Fauna und Flora des 

 Golfs Ton Neapel), Monograpliie, xi. 1884. 



t J. Andreas, " Beitrage zuv Anatomic und Histologie des Sijjunculus 

 nudus,^' Zeitschrift fiir wiss. Zoologie, Bd. xxxvi. 



X Eisig, " Monograpbie der Capitelliden des Golfs von JN'eapel,'' Fauna 

 und Flora des Golfs vun Neapel, xvi. 1887. 



