464 PROFESSOR ALLMAN ON THE RELATIONS OF THE CCELENTERATA. 



Lastly, the Ctenophora (figs. 12, 13) admit of an obvious comparison with a 

 hydroid medusa. In order to understand this, we must keep in mind the pre- 

 sence in the hydroid medusa of an atrial segment of the somatic cavity. This 

 is formed by that portion of the somatic cavity which is immersed in the sub- 

 stance of the umbrella at the base of the manubrium, and from which the 

 radiating canals proceed (fig. 5, V). The hydroid medusa thus admits of a 

 division, by a transverse plane, into two regions : an atrial region (/•), which 

 corresponds to the solid summit of the umbrella with the parts therein con- 

 tained, and a manubrial region (>•'), which corresponds to the manubrium, with 

 that portion of the umbrella which with its associated structures is projected 

 round the manubrium in the form of a bell. 



Now, in a Beroe (figs. 12, 13), the manubrial region is never developed, and 

 the body is represented by the atrial region alone. From the atrium (V V) COll- 



Fig. 10. 



Fig. 11. 





Fig. 10. — Diagramatic longitudinal section of Lucernaria. a, Circum-oral disc ; a!, marginal tentacle ; b, hypos- 

 tome ; V, somatic cavity ; c, aperture by which the chambers of the circum-oral disc communicate with one another 

 across the distal end of the partition ; d, generative bands ; p, peduncle ; z, tentacle-like processes of the inner surface 

 of the somatic cavity. 



Fig. 11. — Diagramatic transverse section of Lucernaria across the circum-oral disc and hypostome. a, a, Cham 

 bers of the disc ; b, hypostome ; d, generative bands. 



tained within this region two radiating canals {a, a) are given off. These imme- 

 diately divide and subdivide, so as to become ultimately eight, which are, 

 moreover, united at their distal extremities by a circular canal, which corre- 

 sponds to that of the medusa, though here thrown back by the non-develop- 

 ment of the manubrial region of the umbrella. Besides the eight longitudinal 

 canals (x, x) into which the two radiating canals ultimately subdivide, these two 

 canals give off, each immediately after its origin, an accessory canal {x x'), 

 which runs without division close to the main body cavity towards the oral 

 orifice, and opens, like the others, into the circular canal. 



The generative sacs (d', d) are developed as diverticula along the course of 

 the radiating canals, whence they extend into the gelatinous substance of the body.' 



