338 To-ansactions . — Zoology. 



The circular layer is external to the transverse, is arranged 

 round the siphons from their apex downwards, and extends 

 for a short distance on to the surface of the body. 



Tlie transverse layer arises in two bauds, one from the 

 apex of each siphon beneath the circular, and spreads out fan- 

 wise on each side over the surface of the body. The muscles 

 of this layer unite to form networks, especially towards the 

 ventral surface. At the anterior end the muscles are very 

 feebly developed, and are covered by a tissue which spreads 

 from the anterior end of the body, and which in spirit changes 

 colour with the gonads. This tissue may spread in scattered 

 patches over the entire surface of the body. I shall refer to it 

 more particularly in speaking of the gonads. 



Into both the oral and atrial siphons are produced delicate 

 lining invaginations of the external coloured surface of the test. 

 In the oral siphon the invagination ends against the circlet of 

 tentacles at the base of the siphon ; in the atrial siphon it is 

 produced for a little way into the interior of the body, hanging 

 loosely in the peribranchial cavity, and from the free edge is 

 produced for a little way a delicate transparent membrane. 

 This arrangement seems to assist in effectually closing the 

 atrial aperture when the animal is above tide-mark, &c. 



C. Digestive Or-gans. 

 A. Tentacles. — The posterior edge of the invagination of the 

 test lining the oral cone ends agamst a stout ring-like pad, 

 bearino; a circlet of tentacles on its lower surface. The ten- 

 tacles are numerous and compound. There are sixteen chief 

 tentacles, placed larger and smaller alternating, but external to 

 these is a large number of smaller tentacles, some of wdiich 

 approach closely in size the smaller tentacles mentioned 

 above. Of the eight large tentacles there is generally one, 

 sometimes two, considerably larger than the others. The 

 one generally much larger is that adjacent to the oral end 

 of the endostyle, while the one which often approaches it 

 very closely in size is that opposite — ^namely, the one adjacent 

 to the dorsal tubercle. In the eight smaller tentacles 

 alternating with the larger, there seems, as in the larger, 

 to be no uniformity in size. In the tentacles external to 

 these I cannot make out any particular arrangement. Each 

 tentacle co]isists of a main axis greatly compressed, concave 

 on the central, convex along the peripheral border. From 

 the lateral border the branches are given off arranged in a 

 double row. From each of the branches are given off 

 branchlets which consist each of a main stem giving off little 

 processes at right-angles. There are also on the main axis of 

 the tentacle, among the branches, branchlets similar to those 

 on the branches. 



