FORMS OF DEEP-SEA ASCIDIANS. 293 



DESCRIPTION OF PLATE XLIV. 



Eigs. 1-6 inclusive illustrate the structure of Hypobythius caly codes. 



Fig. 1. The animal viewed from the ventral surface (i. e. the surface opposite to that on which the nerve- 

 ganglion lies) . The viscera are seen through the transparent body- wall lying on the opposite 

 side of the cavity of the test. At the lateral margins the test swells into a series of lobes 

 composed of masses of compact gelatinous tissue, which lobes give a certain rigidity and 

 support to the test. Similar masses or plates of compact tissue are seen on the ventral 

 surface of the test in its lower region. In the single specimen obtained a longitudinal rent 

 divided the ventral wall of the test, and of the mantle and gill-sacs., along the middle line down 

 to about the point indicated by the letter h. 



This figure represents the animal of half the natural size, and the lettering applies as 

 follows : — 



a, a. Plates of dense test-tissu'e, similar to the other plates also seen, which are unlettered. 

 b. Muscular core of the stem of attachment of the animal, c. Prolongation of test forming the 

 outer tissue of the stem of attachment, here, at the end of the stem, widened out into a funnel- 

 shape, ia. The inhalant aperture. No trace of this aperture remained in the specimen as 

 obtained ; its most probable position only is given. It may have lain somewhat more nearly 

 opposite the mouth on the ventral surface, ea. The exhalant aperture, ng. Nerve-ganglion 

 with sense-organ seen through the test and across its cavity, o. The ovary ; od, the oviduct ; 

 and t the testis, gs. Margin of gill-sac attached by a series of fibres to the membrane lining 

 the interior of the test. Externally to the gill-sacs the directions of the lateral muscular bands 

 and their anterior prolongations along the dorsal margin are indicated, m. The mouth ; s, the 

 stomach ; and h, the heart, bv. Large blood-vessel leading from the heart towards the stem. 



Fig. 2. Sketch of the upper portion of the animal, much reduced in size, and as viewed from the dorsal 

 aspect, to show the symmetrical arrangement of the plates of thickened test-tissue. 



ea. The exhalant aperture, d. Set of four plates covering the generative and digestive 

 organs. Beneath there is a large elongate oval plate. 



Fig. 3. Portion of the gill-sac, to show its small and irregular fenestrations, which have a diameter of 

 about - 10 millim. 



Fig. 4. Bodies, partly crystalline or with crystalline contents, occurring in the tissue of the test. 



Fig. 5. Ovum from the ovary; actual diameter 1*5 millim. 



Fig. 6. Spermatozoa, one of them viewed sideways, so as to show the attachment of the vesicle to the head. 

 Actual length of the heads of the spermatozoa -005 millim. 



Eigs. 7-13, inclusive, illustrate the structure of the Ascidian Octacnemus bythius, 

 trawled March 1, 1875, from 1070 fathoms. 



Fig. 7. The animal viewed from below : natural size. The central part of the figure is occupied by the 

 flat irregularly oval base, with undulating outline and thickened margin (b). Beyond the 

 margin of the base the eight large conical processes are seen to radiate outwards, and terminate 

 in (t) abruptly narrowed tentacle-like tips. The short pedicle of attachment (pa) projects 

 from the anterior part of the base ; the other parts (nucleus, respiratory membrane, and muscles) 

 are seen through the transparent wall of the test constituting the base. The nucleus is turned 

 over slightly to the left hand. 



ea. Exhalant aperture, b. Thickened border of the base. pa. Pedicle of attachment. 

 ' r. The rectum, n. The nucleus, cm. Circular muscles, rm. Radiating muscles. ^.Thickened 

 horizontal part of mantle-membrane. 



