Miscellaneous. 151 



tally of the adult iu a note treating of the development of the 

 embryo of the Acoela, says : — " I have found the nervous sy^stera in 

 the adidt Accela, as v^^ell as the digestive cavity, perfectly visible in 

 sections." All our knowledge upon this subject is limited to this 

 phrase, which is not followed by any description or accompanied by 

 any figure. 



In one of our most interesting Acoela, Convoluta Schultzii (0. 

 Schm.), I have discovered a very developed nervous system, and I 

 have been able to' display it with the greatest distinctness, not only 

 in sections, but in the animal when entire. 



Nervous System. — Around the otocyst we find a bilobate gangli- 

 onic mass, which forms the principal part of the central system. 

 Two other masses, forming a pair, smaller and situated higher up *, 

 are attached to the principal mass by two large connectives, and are 

 united to one another by a transverse commissure. These central 

 parts of the nervous system are composed of fibres and cells. The 

 fibres occupy the centre of the dilated parts, and form almost the 

 whole of the connective and commissural cords. They are exceed- 

 ingly fine and delicate, undulated and parallel. The cells are situ- 

 ated at the periphery of the dilated parts, and form especially a great 

 aggregation at the postero-inferior part of the principal mass, and a 

 continuous layer around the otocyst. They are of an average 

 diameter of 5 ^ to 7 /i, and are polyhedral. In some of them one 

 can see starting from the angles processes which throw themselves 

 into the layer of fibres. Their single nucleus, which is not nucleo- 

 lated, is 3-4 ^ in diameter. 



The peripheral system is formed by six parallel longitudinal 

 nerves and their ramifications. These nerves are situated immedi- 

 ately underneath the layer of zoochlorellae; they form three pairs — an 

 external one, which runs in the folded margins of the body ; an 

 internal one, which descends a little beyond the median line ; and 

 an intermediate one, situated at nearly equal distances between the 

 two preceding. These last two pairs correspond to the four clear 

 streaks which may be observed in the living animal without any 

 preparation. The internal nerve on each side originates from the 

 principal ganglionic mass which surrounds the otocyst. The two 

 external nerves originate by a short, common, transverse trunk 

 from the small superior mass. A cord starting from the inferior 

 ganglion joins the median nerve at its origin, so that the latter has 

 a double origin. These longitudinal trunks are united by transverse 

 anastomoses, which cut them at right angles, like the rungs of a 

 ladder. These anastomoses are not all perfectly constant in their 

 position, but the variation is not considerable. In a general way 

 they become more and more numerous the further they are from the 

 head. At the inferior extremity the cords converge and resolve 



* As usual I place the animal with the head upwards aud the ventral 

 surface in front. 



