ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
41 
Hirudinea differs from that of Nematodes only in the fact that the con- 
tractile substance completely surrounds the medullary mass, so that the 
cell has not the form of a groove, but a closed tubule, which grows 
narrower at each end. Both kinds of cells show a great resemblance to 
the striated muscular fibres of Arthropods and Vertebrates in so far that 
in all the muscular columns are surrounded by the sarcoplasm ; but 
there is this difference, that in Nematodes and Hirudinea the columns 
appear in the cortex, while in Arthropods and Vertebrates the whole 
thickness of the fibre is equally developed ; to this last rule there are, 
however, exceptions. With regard to Chastopods, their muscular cells 
are formed on the same type as that of the Hirudinea, and the same is 
true of Molluscs. The nerve-fibres are, as a rule, tubes of the same 
thickness for their whole length, but they exhibit extraordinary varia- 
tions in thickness ; in all cases an axis-cylinder and a sheath can be 
distinguished ; the former consists of a finely fibrillar spongioplasm 
with a homogeneous hyaloplasm imbedded in it. The ganglionic cells, 
which generally belong to the bipolar or multipolar type, have a coarse 
fibrillar spongioplasm. The nerves of Nematodes are essentially dis- 
tinguished from those of the higher Worms by the absence of dotted 
substance. 
The innervation of the muscle-cell is effected by its medullary sub- 
stance, the presence of which forms an integral part of it. But the 
nerves and muscles are not only closely connected with one another, but 
also with the subcuticle. This layer has not a cellular structure, but forms 
a continuous protoplasmic mass in which nuclei appear to be regularly 
distributed. Fibrils are to be seen in the mass, where they form a close- 
meshed plexus or form parallel bands, circular, longitudinal, or radial 
in direction. In the Ohsetopoda and Hirudinea the fibrous tissue of the 
subcuticle and the spongioplasm of the nervous system are also closely 
connected ; in them, as in Nematodes, the sheaths of the nerve-fibres 
are only a product of the close plexuses of the fibrous system of the 
subcuticle, with which they are often in close connection ; in Nematodes, 
indeed, the passage of spongioplasm into axis- cylinder is so gradual, 
that it is not possible to make a separation between them. 
Nerve, muscle, and subcuticle are found to be in unbroken connection 
by means of their spongioplasm, and the last is seen to play a part in 
innervation ; its thick radial fibres, which stain very intensely, extend 
from the cuticle directly to the inner margin of the median line, where 
they unite into a very complicated network, and are thus brought into 
relation with the muscular processes. 
With regard to the connection between sensory and motor nerve- 
endings, it is suggested that the stimulus exerted from without on the 
papilla is conveyed directly to the bursal ganglionic cell, whence it is 
conveyed by a second process to the bursal nerves ; this, again, is a 
direct continuation of the motor ventral nerves ; and it conveys the 
stimulus to the dorsal median nerve by means of numerous connecting 
fibres which lie in the subcuticle. 
Muscle and Nerve in Mermis and Amphioxus.* — Dr. E. Rohde 
gives a short account of the relations of muscle and nerve in each of 
* SB. K. Akad. Berlin, 1892, pp. 659-64. 
