122 



THE VOYAGE OF H.M.S. CHALLENGE!*. 



test, and terminal expansions on the outer surface are very rare. In different parts of the 

 test, however, usually deeply situated, there are large cavities or reservoirs full of yellowish- 

 brown blood-corpuscles, and appearing to the eye as minute brown dots scattered thickly 

 nver the test. In these species there are also found, in connection with the terminal twigs 

 of the vessels in the superficial layer, numbers of minute thin-walled finger-like processes 

 projecting beyond the surface of the test. They are found in the hollows and grooves on 

 the uneven surface, probably on account of the protection afforded to them in these spots. 

 These delicate processes doubtless represent the papillae of Culeolus murrayi. 



In Culeolus perlucidus, the pouches or cavities filled with blood-corpuscles seen on the 

 inner surface of the test are undoubtedly prolongations from the blood-sinuses of the 

 mantle. No further development of the vascular system in the test was seen either in this 

 species or in Culeolus recumbens. In Culeolus moseleyi and Culeolus perlatus, of each of 

 which there are only single specimens, the test could not be examined. 



The peduncle probably contains blood-vessels in all the species. In Culeolus murrain 

 and Culeolus recumbens the vessels are numerous, and form a network traversing the 

 peduncle. In Culeolus perlucidus there is a single central canal, apparently vascular. 

 In the remaining three species the peduncle could not be examined without injuring 

 the single specimens. In two of them, Culeolus moseleyi and Culeolus perlatus, a net- 

 work of brown bars is distinctly visible from the exterior traversing the substance of the 

 peduncle. These bars possibly indicate thickened or modified portions of the matrix 

 separating canals in which the blood-vessels lie, as seen in Culeolus murrayi. 



The structure of the mantle is very similar in the six species. 



The Branchial Sac. — This is much the most characteristic organ in the genus, and is 

 very similar in all the species. 



Fia. 14. — Part of the Branchial Sac of Culeolus wyville-tlwmsoni, from the inside. 

 lr., large transverse vessel ; tr'., smallest size of transverse vessel ; i. I., internal longitudinal bar ; mh., mesh ; sp., spicula. 



