$$ 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 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 
are large, as is the case with Solen, Mactra, Ve- 
nus, and Cytherea, their two nervous trunks have 
several ganglionic enlargements along their 
course, connected by transverse filaments; see 
Blanchard, loc. cit. p. 333, Pl. XII. fig. 1, 2, d. 
(Solen and Mactra).* 
1 With the. simple Ascidiae, as a sympathetic 
system may perhaps be considered the ganglion, 
which, according 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, Duvernoy, and Blanchard have seen 
the filaments, which issue from the principal ganglia, 
enter the vegetative organs ; but as they could 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 
*[§ 183, note 11.] See Quatrefages (Mém. 
sur le genre Taret. in Ann. d. Sc. Nat. 1849, XI. p. 
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 organic nerves. 
If this is so, the same signification would be 
given to the nervous filaments which Blanchard 
(loc. cit. p. 335, Pl. XII. fig. 1, e.) has seen arise 
with an Arca, and a Solen, from the two small 
ganglia which belong to the commissures of the 
Par posterius. More profound researches upon 
the destination of their nerves, must determine 
whether the two ganglia situated between the labiat 
ganglia, with the apodal Lamellibranchia (see above 
§ 183 note 7), really correspond to the Par pedale, 
or do not rather belong to the sympathetic system. 
63, Pl. I.), who has described in detail this system 
with the Teredina. — Ep. 
