196 



TUE ACEPHALA. 



«§>S^ 181, 182, 183. 



diK'tor muscle of the other valve<^' passes, in order to be attached to for- 

 eign liodics 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.'-' 



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 

 witli one of the others, jnisses into the fissure of the 

 other valve ; wliile the third is inserted upon the 

 same valve. 



- It has already been remarked ( § 177), that with 

 the Brachio|)oda the ])cduncle receives mus- 

 cles both from the body and fro.a the valves. It 

 has, moreover, muscles of its own, anil 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. E.\- 

 ternally, it is composed of a thick cartilaginous tis- 

 sue, while its interior is occupied by a hollow, 

 muscular cord, composed of longitudinal filaments ; 

 see Oicfn, loc. c\t. (Terebratu/a), and Fog-f, loc. 

 cit. Tab. I. fig. 1-6 (Linsrula). 



1 These orange-colored ganglia aro quite distinct 

 with Unio, and Anodonta. 



'■i 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. 



