120 THE VOYAGE OF H.M.S. CHALLENGER. 



The Tentacles are small, and seem not to differ strikingly in size. They are very 

 delicate, and are very slightly branched, almost simple, having only occasional minute pinnae. 

 Calcareous spicules similar to those of the branchial sac are plentiful in the tentacles. 



Tlie Dorsal Tubercle is rather large, and has more of the usual appearance than those 

 of Guleolus recumbens and Culeolus perlucidus. It is of a transversely ovate shape, with 

 the aperture turned towards the right side (PI. X. fig. 12). The anterior horn turns out- 

 wards and upwards, while the posterior horn, also turned outwards, is coiled in a close 

 spiral of one and a half turns. The peritubercular area is triangular and not large in 

 comparison with the size of the tubercle. The peripharyngeal bands forming its lateral 

 boundaries are straight (PI. X. fig. 12, p.p.). 



The Nerve Ganglion. — The ganglion is large and of a bright yellow colour. It has an 

 oblong quadrangular shape (PI. XIII. fig. 4, n.g.), and gives off two large nerve trunks at 

 each end. 



In the mantle covering the ganglion dorsally, there is a patch of branched calcareous 

 spicules, similar to those in the branchial sac. They are very numerous, and cross 

 and interlace, forming in one part of the patch a very dense reticulum (PI. XIII. 

 fig. 4, sp.). 



The single specimen of this species is in a good state of preservation. It was obtained 

 in the centre of the Pacific Ocean, almost on the Equator. 



Station 271. September G, 1875; kit. 0° 33' S., long. 151° 34' W. ; depth, 2425 

 fathoms; bottom temperature, 1° C. ; globigerina ooze. 



Comparison of the Species of Culeolus. 



As the six species of this genus differ in nearly all the minute details of structure, it 

 will be interesting to go over the more important organs, and point out the differences 

 and resemblances which they present in the different species. 



External Appearance. — In respect to external appearance there is a strong general 

 resemblance between the species. They all consist of a more or less ovate body borne 

 on a long peduncle. In all, the a)iterior end of the body, where the peduncle is attached, 

 is narrower than the posterior end, and in none of them is there much lateral com- 

 pression. 



In all the species the peduncle, after leaving the test, turns at a right angle — in two of 

 them (Culeolus recumbens and Culeolus perlucidus) ventrally, and in the other four 

 dorsally. Consequently, in all the long or antero-posterior axis lies at right angles to 

 the peduncle, and therefore in a more or less horizontal position. In Culeolus wyville- 

 thomsoni, and Culeolus perlucidus this axis comes to be inclined downwards and 

 posteriorly on account of the curvature of the peduncle, while in Culeolus recumbens 

 the peduncle is so flexible that it could not have supported the weight of the body, which 

 must, therefore, have rested on the bottom. 



