Anatomy and Histology of Nemer tines. 397 
displays a marked conformity with that of Groups I. and II. 
in that it likewise consists of circular and longitudinal layers 
of fibres, in addition to which, in the case of many Rhabdo- 
coela, we also have a diagonal layer, lying between the two 
former. Much 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 Cerebratulus in particular. 
I have alluded to the fact that the integument, and 
especially 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 Nemertines 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 Andress j", who states that it contains 
pigment-masses and gland-cells. The phenomena presented 
by the hypodermis of the Annelids and the ectoderm of the 
Nemertines 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 
Capitellidse) $. The musculature of the body-wall of the 
Annelids is allied to that of Groups I. and III., since it con- 
sists of a circular and a longitudinal layer. If we neglect 
the fact that the diagonal layer, which is stated by Andreas 
to lie in Sipunculus between these two muscle-layers, does 
not entirely agree in structure with that of the Nemertines, 
the musculature of the body-wall of a Carinella or a Drepa - 
nophorus would be essentially the same as that of the 
Gephyrean. 
In all Nemertines 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 de3 Golfs von Neapel ” (Fauna und Flora des 
Golfs von Neapel), Monographic, xi. 1884. 
t J. Andrese, “ Beitrage zur Anatomie und Histologie des Sipunculus 
nudusf Zeitschrift fiir wiss. Zoologie, Bd. xxxvi. 
X Eisig, u Monographic der Capitelliden des Golfs von Neapel/’ Fauna 
und Flora des Golfs von Neapel, xvi. 1887. 
