ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 199 



physiologically, most necessary to it ; by means of these partly 

 provisional organs, the larva seeks its food, and the material for 

 afterwards developing its body; in other words, the trochosphere 

 does not represent the whole body of a Platyhelminth, but merely 

 the cephalic portion of Gunda, with a fresh structure, the anal 

 segment. 



Echinodemiata. 



Nervous System of the OpMuroidea.* — N. Apostolides, who 

 has already I examined and described the circulatory and respiratory 

 organs of this little-studied group, says that the circum-oral nerve- 

 ring is contained in a " perineural " space which forms part of the 

 body-cavity, and communicates with the general body-cavity at the 

 point of entrance of the radial nerves into the arm ; the space is 

 triangular in transverse section, and is bounded on the outer side 

 by the second discoid ossicle of the skeleton, and above and below 

 by two membranes which originate in the point of union of the 

 stomach and oesophagus, and are inserted on the ossicle. The nerve- 

 ring itself forms a vertically flattened band. The radial nerves pass 

 off from it horizontally, each traversing the foramen in the second 

 discoid ossicle ; each then turns upwards as far as the ventral plate, 

 when it again becomes horizontal and then traverses the furrow of 

 the arm. The annular canal of the water-vascular system and its 

 branches lie outside the corresponding parts of the nervous system. 

 Histologically, the nerve-band consists of two distinct tissues, the one 

 ventral, consisting of brown cells with large nuclei and not coloured 

 by picrocarmine ; they have been wrongly regarded by most writers 

 as constituting the essentially nervous element, but they resemble 

 rather the pigment-cells of Vertebrata. The dorsal portion of the 

 band is the true nervous part ; it forms a very small portion of the 

 whole band, and lies in a groove on its superior aspect ; it consists 

 of extremely delicate fibrils, between which pale bipolar cells lie 

 scattered, not aggregated into ganglia. 



The radial nerves exhibit certain dilatations opposite to intervals 

 between the ossicles, but they are composed of the same non-nervous 

 matter as that of which the ventral part of the ring consists. No 

 branches are given off by the central ring, but the radial nerves 

 give off from their origin a pair of nerves, the upper one of which 

 goes towards the first tentacle and, when near it, divides; the two 

 twigs thus formed course round the end of the tentacle and meet on 

 the opposite side of it. The exact distribution of the nerve in the 

 walls of the tentacle is unknown. The lower of the branches of the 

 radii goes towards the muscles which lie between the angles of the 

 mouth. Two similar pairs of nerves are given off by the radial 

 nerve before it reaches the arm and another pair within the arm, 

 all having the same distribution as the first pair. 



American Comatulse.l — Iii his preliminary report on these forms 

 Mr. P. H. Carpenter states that he thinks he has discovered as many 



* Comptes Kendus, xcii. (1881) pp. 1424-6. 



t See this Journal, i. (1881) p. 466. 



t Bull. Mus. Comp. Zool. Camb., ix., No. 4 (1881) 20 pp. (1 pi.). 



