ZOOLOGY AND BOTANY, MIOllOSCOPY, ETC. 501 



Structure and Metamorphosis of Larva of Flustrella hispida.* — 



M. n. Proiilio has made a study of tlie larva of this Bryozoou. In the 

 oral region there is a pyriform organ with a vibratile pluine and a 

 sucker, and in the aboral region an ectodermic pad ; the two regions are 

 scjiarated by a ciliated corona. In addition to these there is an internal 

 hollow organ, the cavity of which communicates with the exterior by an 

 orifice on the oral surface between the pyriform body and the sucker ; it 

 may be regarded as an embryonic digestive sac. As the larva approaches 

 the free stage the walls of the sac become less and less distinct, and are 

 finally absorbed. In this poiut the free larva differs essentially from 

 Cijphonautes, in which, as all agree, there is a digestive tube. The 

 ectoderm and mesoderm become considerably differentiated in the free 

 larva. A bundle of nerve- fibres, with which are connected some unipolar 

 cells, directly connects the pyriform organ with the aboral pad. Some 

 of these fibres extend as far as the vibratile plume and make their way 

 between the glandular cells of the pyriform body, while a right and a 

 left bundle become detached to furnish fibres to the ciliated cells of the 

 groove, to those of the corona, and to the very numerous vibratile swel- 

 lings which are scattered over the dorsal surface. As all the ciliated cells 

 of the larva are connected w^ith the aboral ectodermic pad, and as this is 

 provided with rigid cilia, it appears to be justifiable to regard it as 

 having a sensory function. 



On each side of the larva there are parietal muscles comparable to 

 those of the adult, and longitudinal muscles, which are adductors of the 

 valves, traverse the middle of the larva. The most interesting meso- 

 dermal structure, and one which has not yet been noticed in the larvae of 

 marine ectoproctous Bryozoa, is a subepidermic cellular layer w^hich is 

 particularly developed in the aboral region. 



When the larva becomes fixed the corona is folded inwards, and the 

 sucking plate fuses with the skin all round the free edge of the valves ; 

 the changes which now occur agree with what are seen during the 

 fixation of a cheilostomatous larva. The corona, pyriform body, nervous 

 system, and a portion of the musculature then undergo degeneration, 

 and form a mass of globules enveloped by the mesodermic layer. The 

 thickened jjlate of the ectoderm soon afterwards proliferates rapidly, 

 and forms an invagination below the cuticle, which does not itself take 

 part in it. 



Arthropoda. 



Segmental Sense-Organs of Arthropods.j — Mr. W. Patten states 

 that the cephalic lobes of Acilius are comjiosed of three segments, each 

 of which contains a segment of the brain, oj^tic ganglion, and optic jjlate. 

 It is very i)robable that these characters are common to all Insects. 

 The segmental nature of the eyes is more clearly seen in the embryos of 

 scorpions, spiders, and Limulus, where it can be shown that they are 

 serially homologous with one or more pairs of sense-organs on each 

 segment of the thorax. If the cephalic lobes of scorjDions could be 

 stretched out the eyes would lie, as in Acilius, on the thickened 

 outer edge of each segment. This thickened edge is represented in the 

 post-oral region of the pleurae of the thoracic segments, each of which 

 bears two large sense-organs close together near the outer edge of the 



* Comptcs Rcndas, cviii. (1889} pp. 1023-5. 

 t Journal of Blorpliology, ii. (1889) pp. eOO-2. 



