196 $$ 181, 182, 183. 
ductor muscle of the other valve™ passes, in order to be attached to for- 
eign bodies by its smooth, calcareous extremity. 
With the Brachiopoda, there is a real peduncle which constantly pro- 
jects through an opening near the hinge. It is a soft tendinous or muscu- 
lar tube, which is, perhaps, only a prolongation of the mantle.® 
THE ACEPHALA. 
CHAPTER III. 
NERVOUS SYSTEM. 
§ 181. 
The nervous system, which has been observed in all the orders of the 
Acephala, consists of a central and a peripheric portion. 
The first is composed of one, or several (usually three) ganglia ; the second 
consists of nervous trunks of variable size, which pass off in the most dif- 
ferent directions. When the number of these ganglia is considerable, they 
are arranged in pairs which are situated more or less near the median line, 
according to the different regions of the body. The ganglia of each pair 
intercommunicate by a transverse commissure of variable length. They 
connect, moreover, with others, which are even far removed, by anastomosing, 
filaments. It is difficult to decide which of these ganglia is the brain. Many 
species want a complete ganglionic ring surrounding the buccal cavity. 
§ 182. 
From its extreme softness, the internal structure of the nervous system 
of the Acephala is very difficult of study. Its primitive fibres are very 
delicate, and are surrounded, in the nervous trunks, by a distinct and very 
thin neurilemma. In the ganglia, through which orange-colored granules. 
are usually scattered,” these fibres pass into a very loose tissue composed 
of small transparent vesicles, which probably take the place of the gan- 
glionic globules which are so distinct with other invertebrates. 
§ 183. 
1. The nervous system is most simple with the Tunicata. It here consists 
of a single ganglionic mass, which is subcutaneous, and situated between 
the two respiratory tubes. 
1 Carefully examined, Anomia will be found to 
have three unequal adductor muscles arising from 
the imperforate valve. The largest of these, together 
with one of the others, passes into the fissure of the 
other valve ; while the third is inserted upon the 
same valve. 
2 It has already been remarked (§ 177), that with 
the Brachiopoda the peduncle receives mus- 
cles both from the body and from the valves. It 
has, moreover, muscles of its own, and ought, 
therefore, to be contractile. This contractility is 
quite prominent with the very large and long pe- 
duncle of Lingula, especially in comparison with 
the very short, sucker-like one of Orbicula. Ex- 
ternally, it is composed of a thick cartilaginous tis- 
sue, while its interior is occupied by a hollow, 
muscular cord, composed of longitudinal filaments 5 
see Owen, loc. cit. (Terebratula), and Vogt, loc. 
cit. Tab. I. fig. 1-6 (Lingua). 
1 These orange-colored ganglia are quite distinct. 
with Unio, and Anodonta. 
2 Although the Naiades have very large ganglia, 
yet their microscopic examination has furnished no 
further results, for neither by the compressorium, 
nor by chemical means, can these globules be sep- 
arated from the intervening tissue. 
