MUSEUM OF COMPARATIVE ZOOLOGY. 61 



and Crisia. The existence of two layers was easily demonstrated in all 

 cases in the young polypide by cross sections of the " neck." The two 

 layers are of nearly the same thickness, and distinctly separated from 

 each other. The presence of two layers in the adult is more difficult to 

 determine, but it was always indicated by the occasional presence of 

 two nuclei lying side by side, and especially at the attachment to the 

 diaphragm. The presence of muscles was demonstrated in all cases 

 (except Bowerbankia, where my few sections did not show the proper 

 region) upon tangential sections of the sheath. I may add, that the 

 existence of muscles is wellnigh conclusive a priori evidence of the 

 existence of the mesodermal layer, since nowhere else in Bryozoa, so far 

 as I know, do muscles arise from any other layer. Prouho's evidence in 

 support of his position is perfectly satisfactory to my mind, certainly 

 more so than the negative evidence of Seeliger in support of his. In 

 further support of my statements I may refer to the condition of tlie 

 kamptoderm (kmp^drm.) in Figures 92 and 83, Plate X. 



Nervous Si/stem. — Since Dumoi-tier discovered, in 1835, a ganglion in 

 Lophopus, there has been seen in marine as well as fresh water Bryozoa 

 a body which has been considered, with greater or less certainty, to con- 

 stitute the central nervous system. Oveidooked by Farre, it was, I be- 

 lieve, first described for marine Gymnolremata in 1845 by van Beneden, 

 co-worker with Duraortier, for Laguncula (Farrella). Nevertheless, up 

 to the present the evidence of its being a ganglion homologous with that 

 of Phylactoleemata has not been satisfactory. The homology can be estab- 

 lished only by determining its similar origin with the brain of Pliylacto- 

 Isemata ; its function can be best established by showing the existence of 

 ganglionic cells and fibres. I hope to have advanced our knowledge in 

 both of these directions. 



At about the time that the oesophagus and stomach have become con- 

 fluent, one notices a papilla-like elevation of the floor of the atrio-pha- 

 ryngeal cavity. This has been noticed by Korotneff" ('74) in Paludicella, 

 and by Nitsche ('71, p. 459) and Seeliger ('90, p. 586) in Cheilostomes. 

 It has been called by them " Epistome," and compared with that of Endo- 

 procta or Phylactolaemata. In my own opinion, it is merely a structure 

 brought into prominence by the sinking down of the floor behind it to 

 form the ganglion (Plate X. Fig. 86, gn!). This depression has been 

 seen by BaiTois ('86, pp. 74, 75) and Prouho ('90, p. 450), and rightly 

 interpreted by them as probably destined to gi^je rise to the central 

 nervous system. That this is the correct interpretation is shown by 

 later stages from different species, as Figures 89 and 83, in which we see 



