20 LUCERNARI.E AND THEIR ALLIES. 



evidout by seeing its margin follow all the various shapes which are assumed by 

 that of the front face, but unless so affected it docs not appear ever to change its 

 contour. As we sliall see hereafter, in detail, it has no muscular component, but 

 is simply resilient when bent or compressed. In this respect its junction with the 

 front face is very abrupt, but in point of configuration is comparatively gradual, 

 forming with the other a smooth rounded edge {_figfi. 61, 62). 



46. Since the three main subdivisions of the body have a close relation to the 

 disposition of the various members of the organization, one would be justified, upon 



.these premises, in looking for something marked at the point of transition from the 

 unibella to the peduncular, caudal regions (t). This presumption, however, is not 

 warranted so far as the exterior surface is concerned, for we find there a very 

 gradual transition in point of external features, but a rather abrupt change of general 

 form. We must look within for more evident diversities between these two regions. 

 There is no visible, structural dividing line which separates them, as there is 

 between the front and the posterior faces of the umbella, and therefore it is not 

 possible to say where the one begins and the other ends ; the most that we can 

 state is that tlic bulk of the posterior umbellar face is convex, but that behind this 

 it rapidly curves oft' into a more or less broad, conical form, and then insensibly 

 makes a transition into the peduncle. 



47. The interior of the umheUa next demands our attention. In regard to its 

 general configuration it miglit with propriety be described as the mould of that of 

 the exterior. We have already (43) indicated its subdivision into four compart- 

 ments, when referring to the four partitions which unite the anterior and posterior 

 faces of this region, but we have further details to add here, and shall therefore 

 begin Avith the partitions as the foundation for the principal modifications of the 

 general cavity of this part of the body. The anterior {^figs. 22, 37, GG, 'Q and pos- 

 terior (/3) parietes of the umbella are united by their interior faces at four equidis- 

 tant points, or rather lines (\]/-), which from without have the deceptive appearance 

 of tubes, on account of their comparative transparency. These extremely elongate 

 areas of attachment, or of mutual fusion, extend from each of the bases of the four 

 flat sides of the proboscis to within a short distance of the margin of the umbella, 

 and then abruptly terminate. Consequently at each of these four points of termi- 

 nation there is left a passage-way (4/) from one compartment to the other, and 

 therefore the subdivisions of the general cavity intercommunicate at their distal 

 ends, as well as at the proximal apertures behind the base of the proboscis. 



48. These partitions can scarcely be called division Avails, as tliey have barely an 

 appreciable depth, but are rather to be compared to low ridges on two opposing 

 surfaces which have inosculated along their crests. By making a transverse section 

 {fig- 61) across two approximated arm-like angles of the octagon, the sliglit depth 

 of the partitions (^=) can be very clearly demonstrated. We should not fail here 

 to state their exact topographical relations, since they constitute an important 

 clement in the morphological construction of the body, both as regards its ordinal 

 characters, and in the consideration of its embodiment of the typical idea of the 

 grand division to which it belongs. The vertical and horizontal axial planes being 

 understood to lie in continuation of the four diagonally opposite corners of the 



