ACTIXOZOA. 225 



and that each half of the long unpaired marginal 

 canal is homologous with one of the lateral oral 

 vessels in such genera as Bolina or LeSueuria. 



Two principal kinds of tentacles occur in the 

 Ctenophora: long, highly contractile cords, capable 

 of being retracted into special pits ; and shorter, 

 isolated threads, which may, in some species, 

 become aggregated to form tufts or bunches. 

 Among the BeroidcB, tentacles are absent. In 

 Pleurobrachia a large tentacular pit excavates 

 obliquely upwards the substance of the two lateral 

 actinomeres. The base of this pit is brought into 

 close connection with the distal extremity of the 

 short primary radial canal, which opens directly 

 into a wide heart-shaped sac, from between the two 

 deeply-cleft lobes of which, at the upper portion of 

 the fissure formed by their junction, the tentacle 

 itself makes its appearance. Proximally, it is some- 

 what compressed, but for the greater part of its 

 length becomes truly cylindrical, giving off on that 

 side which is turned away from the body a number 

 of secondary lateral filaments. Both these and 

 the tentacle itself are hollow, communicating 

 with the canal system through the medium of 

 the basal sac, their walls also, like those of the 

 body canals, being lined by an investment of 

 endoderm. On the secondary branches them- 

 selves still more minute threads are said to have 

 been observed. Of the grace and beauty which 

 the entire apparatus presents in the living animal, 

 or the marvellous ease and rapidity with which it 

 can be alternately contracted, extended, and bent 

 at an infinite variety of angles, no verbal descrip- 

 tion can sufficiently treat. These movements seem 

 partly caused by the action of the contractile fibres 



Q 



