§S^ 184, 185. 



THE ACEPHALA. 



199 



The siphon and its muscular apparatus receive their nerves also from this 

 same pair. *" 



The nerves of the Par inferius being destined chiefly for the foot, cor- 

 respond in number and size with the degree of development of this organ. 

 This number, however, varies between two and six for each side. 



■§ 184. 



The Acephala have, certainly, a Splanchnic nervous system, but as yet it 

 has been found only with the Lamellibranchia;'^' and even here it is 

 seen with difficulty and imperfectly on account of the extreme tenuity of 

 its filaments. 



With some species, delicate, lateral filaments pass off from the nerves of 

 communication, which connect the Par gangliorum inferius and posterius 

 with the Par anterius ; these may be properly termed sympathetic nerves, 

 for they are distributed partly to the walls of the digestive canal, and the 

 heart, and partly to the liver, the gland of Bojamcs, and the genital or- 

 gans. ^^> 



CHAPTEK IV 



ORGANS OP SENSE. 



§ 185. 



Of the organs of sense with the Acephala, those of Touch arc the most 

 highly developed. They usually consist of conical, or flattened, protractile 

 prolongations of the skin, which are extremely irritable, covered with cili- 

 ated epithelium, and often of a deep color. 



11 When the two retractor muscles of the siphon 

 are large, as is tlie case with Solen, Mactra, Ve- 

 nus, and Cytkerea, their two nervous trunks have 

 several ganglionic enlargements along their 

 course, connected by transverse filaments ; see 

 iilanchard, loc. cit. p. 333, PI. XII. fig. 1, 2, d. 

 (_So/en and Mactra).* 



1 With the simple Ascidiae, as a sympathetic 

 system may perhaps be considered the ganglion 

 which, accorcling to Schalk (loc. cit. p. 9, fig. 4, g, 

 q.) is concealed between the intestinal convolutions, 

 at the posterior extremity of the body of Phallu 

 sia, and send off filaments in various directions 

 But, as yet, the existence of this ganglion needs 

 confirmation. 



2 Garner, Duvernoij, and B/anchard have seen 

 the filaments, which issue from the principal ganglia, 

 enter the vegetative organs ; but as they c.uld not 

 further trace them, they hesitate to regard them as 

 organic nerves. Keber is more positive in favor of 

 the existence of a sympathetic system with the 



Lamellibranchia. He has observed (loc. cit. p. 

 15) that the commissural filaments, which pass 

 into the Par posterius, give off branches to the 

 intestinal canal, to the liver, and gland of Bojanus; 

 and that those of the Par pedale give off similar 

 branches to the genital organs ; and also, that 

 these nerves form several Plexus between these 

 organs, and from which are given off filaments to 

 the heart. From this disposition, he ought to con- 

 clude that these are real w'ganic nerves. 



If this is so, the same signification would be 

 given to the nervous filaments which Blanchard 

 (loc. cit. p. 335, PI. XII. fig. 1, e.) has seen arise 

 with an Area, and a Solen, from the two small 

 ganglia which belong to the commissures of the 

 Par posterius. More profound researclies upon 

 the destination of their nerves, must detennine 

 whether the two ganglia situated between the labial 

 ganglia, with the apodal Lamellibranchia (see abuve 

 § 183 note V), really correspond to the Par pcdate, 

 or do not rather belong to the sympathetic system. 



• [ § 183, note 11.] See Quatrefa^es {fAim. 

 BUT le genre Taret. in Ami. d. Sc. Nat. 1849, XI. p. 



63, PI. I.), who has described in detail this system 

 with the Tcredina. — Eu. 



