54 
POPULAR SCIENCE REVIEW. 
the ovarial tube or immediately after their extrusion, that the 
first developmental stages are run through rapidly, and that the 
young are passed back from the ovarial opening, which is at 
the side of the mouth, along the dorsal ambulacra, and arranged 
in their places by the automatic action of the ambulacral ten- 
tacles themselves.” 
A still more curious and interesting example of this direct 
mode of reproduction, coupled with the presence of a complete 
incubatory cavity, is furnished by a small Holothuroid which 
was dredged in February in seventy-five fathoms of water at 
the entrance of Corinthian Harbour (otherwise called Whiskey 
Bay) in Heard Island, to which Sir C. Wyville Thomson gives 
the unenviable distinction of being, so far as he knows, 44 the 
most desolate spot on Grod’s Earth.” It belongs to the curious 
genus P solus, of which we have one or two British species, and 
which is characterized by having the rather short body encased 
in calcareous scales and plates, with the exception of a soft disc 
occupying a great part of its lower surface, and bearing three 
rows of ambulacral tentacles, on which the animal crawls, very 
much after the fashion of a Grasteropod mollusc. The mouth, 
with its surrounding ten tentacles, and the vent, open on the 
upper surface, or at least quite separate from the ambulatory 
disc. 
The species upon which Professor Wyville Thomson’s obser- 
vations were made, measures rather more than an inch and a 
half in length, and is considered by him to be nearly allied to 
the northern Psolus operculatus , of which he thinks it may 
possibly turn out to be only a variety. The oral aperture is 
furnished with a small low pyramid, formed of five closely fitting 
calcareous plates, which close it accurately when the mouth with 
its surrounding tentacles is withdrawn within the test (fig. 2) ; 
the anal aperture is closed by a similar but less regular valvular 
apparatus (figs. 2 and 3). 
In the female the middle of the back is occupied by a sort of 
saddle (fig. 2.) composed of large finely granulated calcareous 
plates of rather irregular form, but fitting together with tolerable 
accuracy, and from this character the animal has been provision- 
ally named Psolus ephippifer. It is here also that we find the 
peculiarity which renders this species particularly interesting in 
connexion with the subject at present before us. 44 On removing 
one or two of the central plates,” says Sir C. Wyville Thomson, 
44 we find that they are not, like the other plates of the perisom, 
imbedded partially or almost completely in the skin, but that 
they are raised up on a central column like a mushroom or a 
card-table, expanding above to the form of the exposed portion 
of the plate, contracting to a stem or neck, and then expanding 
again into an irregular foot, which is imbedded in the soft 
