^§ 184, 185, THE ACEPHALA. 199 



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



same pair. '^* 



The nerves of the Par inferhis 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 oif from the nerves of 

 communication, which connect the Par gangliorum inferius and posterius 

 with the Par anterms ; these may be pi'operly 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 Bojanus, and the genital or- 

 gans.'-^ 



CHAPTER IV 



ORGANS OF SENSE. 



§ 185. 



Of the organs of sense with the Acephala, those of Touch are 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 Lamellibranchia. He has observed (loc. cit. p. 



are large, as is the case with Solen, Mactra, Ve- 15) that the commissural filaments, which pass 



»IMS, and Cytherea, their two nervous trunks have into the Par posterius, give off branches to the 



several gangUonic enlargements along their intestinal canal, to the liver, and gland of Bojanus; 



course, connected by transverse filaments ; see and that those of the Par pedale give off similar 



B/anchard, loc. cit. p. 333, PI. XII. fig. 1, 2, d. branches to the genital organs ; and also, that 



(So/era and Mactra).* these nerves form several Plexus between these 



1 With the simple Ascidiae, as a sympathetic organs, and from which are given off filaments to 

 system may perhaps be considered the gangUon, the heart. From this disposition, he ought to con- 

 which, according to' Schalk (loc. cit. p. 9, fig. 4, g. elude that these are real organic nerves. 



q.) is concealed between the intestinal convolutions, If this is so, the same signification would be 



at the posterior extremity of the body of Phallu- given to the nervous filaments which Blanchard 



sia, and send off filaments in various directions, (loc. cit. p. 335, PI. XII. fig. 1, e.) has seen arise 



But, as yet, the existence of this ganglion needs with an Area, and a Solen, from the two small 



confirmation. ganglia which belong to the commissures of the 



2 Garner, Duvernoy, and Blanchard have seen Par posterius. More profound researches upon 

 the filaments, which issue from the principal ganglia, the destination of their nerves, must determine 

 enter the vegetative organs ; but as they could not whether the two gangha situated between the labial 

 further trace them, they hesitate to regard them as ganglia, with the apodal Lamellibranchia (see above 

 organic nerves. Keber is more positive in favor of § 183 note 7), really correspond to the Par pedale, 

 the existence of a sympathetic system with the or do not rather belong to the sympathetic system. 



* [ § 183, note 11.] See Quatrefag'es (M^m. 63, PI. I.), who has described in detail this system 

 sur le genre Taret. in Ann. d. Sc. Nat. 1849, XI. p. with the Teredina. — Ed. 



