598 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
in some of tlie ganglion-cells of Pontobdella , by the median cells, some 
of the fibrils of which form a plexus of thick fibres, and by the structure 
of the ganglionic-cell-processes which pass directly into the nerves. 
The hyaloplasm is the sole nervous constituent ; in consequence of 
its homogeneity and its resistance to colouring matters it is only rarely 
detected in sections between the closely packed fibrils of the spongio- 
plasm ; in teased preparations, however, it is distinctly seen not only in 
the ganglionic cells, but in the central substance of the ganglia, nerves, 
and commissures. 
All the three last-mentioned parts are surrounded by a firm neuri- 
lemma, formed of connective tissue, almost homogeneous in appearance 
and often divided into lamellae. In the ganglia it forms an inner layer 
which separates the central substance and ganglionic cell-layer, and an 
outer which marks off the coelom. In Pontobdella the neurilemma 
remains as an outer layer, while in Aulostomum it makes its way into 
the interior. The radial septa of the commissures have nothing in 
common with the neurilemma, and they are not to be regarded, as they 
generally are, as internal continuations of it. 
Anatomy and Histology of Sipunculus nudus.* — Mr. H. B. Ward 
has made a study of some points in the anatomy of Sipunculus nudus. 
He deals largely with the histology of the body-wall, the tentacular fold, 
and the nervous system. 
Though there is a general similarity to S. Gouldii , correspondence 
in details is wanting ; for example, bicellular dermal glands are entirely 
wanting in the latter, and the multicellular glands of the two species 
are different. Indeed, if the two are to be retained in one genus, some 
modification of Selenka’s diagnosis of Sipunculus is needed. 
In the nervous system of Sipunculids and Annelids there is, it is 
true, a striking similarity, but there are at the same timo certain charac- 
teristic differences. In the former the peripheral system of plexuses is 
very highly developed and consists almost entirely of fibres, whereas 
the dermal plexus of Capitellids, Nemertines, and Polychaata is largely 
composed of ganglionic cells. 
The minute anatomy of the central nervous system of S. nudus bears 
a close resemblance to that described by Rohde for Chastopods and by 
Burger for Nemertines, but the Sipunculids have no “ giant cells ” ; the 
Echiurids, on the other hand, have giant fibres, and it is very probable 
that they are more closely related to the Annelids than are Sipunculids. 
j3. Nemathelminthes. 
Stylet of Heterodera Schachti.j — M. J. Chatin points out that it is 
incorrect to say that this Nematoid of the beetroot and other plants is 
unarmed, for, as a matter of fact, it has a very interesting stylet. This 
is formed of a plate and an apophysis to which muscles are attached. 
Brownish in colour and very elastic the stylet is pierced by a central 
canal ; it is moved by protractor and retractor muscles. The remarkable 
dimorphism exhibited by this worm is seen even in the stylet, for, among 
* Bull. Mus. Comp. Zool.. xxi. (1891) pp. 143-82 (3 pis.), 
t Comptes Rendus, cxii. (1891) pp. 1518-8. 
