196 THE ACEPHALA. * «^«^^ 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.'^' 



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 poi-tion. 



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 the very short, sucker-like one of Orbicula. Ex- 

 have three unequal adductor muscles arising; from ternally, it is composed of a thick cartiUiginous tis- 

 the imperforate valve. The largest of these, together sue, while its interior is occupied by a hollow, 

 with one of the others, passes into the lissure of the muscular cord, composed of lonjjiludinal filaments f 

 other valve ; while the third is inserted upon the see Owen, loc. cit. {Terebratula), and y'ogt, loc. 

 game valve. cit. Tab. I. fig. 1-6 {Lingula). 



2 It has already been remarked ( § 177), that with 1 These orange-colored ganglia are quite disüuct 

 the Brachiopoda the peduncle receives mus- with Unio, and Avndonta. 



Dies both from the body and from the valves. It 2 Although llic Nuiiidu.s have very large ganglia, 



has, moreover, muscles of its own, and ought, yet their micruscnpie examination has furnished no 



therefore, to be contractile. This contractility is further results, f..r neither by the coin|)ressorimn, 



quite prominent with the very large and long pe- nor by chemiad means, can these globules be sep- 



duDCle of Lin^ula, especially in comparison with arated from the intervening tissue. 



