496 Miscellaneous. 



Peltoaaster, which is closely allied to Sacctdina. Having succeeded, 

 not without difficulty, in obtaining some Peltogasters, I have dis- 

 sected them and succeeded in discovering the nervous system. 



The central organ (as in Sacculina) consists of a single ganglion. 

 This ganglion is situated in the sagittal plane in the interior of the 

 mesentery which unites the cloaca to the peduncle, almost between 

 the cement-glands *, but, nevertheless, a little beyond towards 

 the cloaca, exactly at the level of the testicular caecum. Its situa- 

 tion is therefore very superficial, as it is separated from the outer 

 world only by half the thickness of the mantle. It is an elongated 

 ganglion which measures about T \y millim. ; hence it is hardly visible 

 to the naked eye, even when it has been completely prepared by 

 dissection under the microscope with the aid of the erecting prism. 

 It is absolutely simple, and by no means composed of two approxi- 

 mated symmetrical masses. Its constitution includes small, fusi- 

 form, peripheral cells, and large central multipolar cells with a 

 large rounded nucleus furnished with a punctiform nucleolus. It 

 gives origin to numerous nerves, which, however, are very fine and 

 extremely difficult to trace by dissection, since they are hardly 

 distinguishable from the muscular and connective fibres in the midst 

 of which they twist about. On the side of the cloaca it gives off 

 four long filaments, two of which, superficial and forming a pair, 

 run to that organ, and particularly to its sphincter ; while the two 

 others, also a pair, penetrate the visceral mass between the two 

 symmetrical halves of the ovary. From its opposite extremity it 

 gives origin to a large median trunk which insinuates itself between 

 the two symmetrical masses of the ovary, giving off from place to 

 place ramifications to each of them. From the origin of this trunk, 

 or perhaps from the ganglion itself, originate two small nervous 

 filaments, which follow the inner margin of the testes, and are dis- 

 tributed to those organs. Upon the sides originate three pairs of 

 nerves — two in the cloacal half of the body, one in the opposite half. 

 The former two run obliquely outward, and penetrate into the 

 mantle at the point where this splits to form the lamellae of the 

 mesentcrv. The last passes between the cement-gland and the 

 testis, then outside the latter, taking a direction towards the defe- 

 rent canal. Near its origin it furnishes two branchlets for the 

 cement-gland, one for the gland itself, the other for its lateral parts, 

 and no doubt for its sphincter. Not far from its termination it 

 o-ives off a nervous filament, which passes under the margin of the 

 mesentery, to run, no doubt, to the wall of the visceral mass or to 

 the mantle. 



Just as these nerves are difficult to trace, so are the central gan- 

 glion and the origin of the principal trunks easy to see, when one 

 knows where to look for them ; but this last condition is indispensable. 



* The cement-glands, which have been regarded as absent, really 

 exist ; but they are reduced to two very wide sacs with glandular walls, 

 but without ramifications. 



