30 THE VOYAGE OF IOl.S. CHALLENGES. 



The Mantle (The Second, Inner, or Muscular Tunic, Muscular Sac). 



The second layer covering a Tunicate is the muscular and connective tissue part of 

 the body wall, and lies immediately inside the ectoderm or epidermis. It varies greatly 

 in its characters in different groups, and is to a certain extent a reliable, distinguishing 

 feature, especially for families and genera. It is formed of connective tissue, uniting and 

 enclosing bundles of muscular fibres, nerves, and blood-vessels. In some cases numbers 

 of the connective tissue corpuscles become pigmented, so as to give a coloured or 

 variegated appearance to the mantle. 



In the living animal the mantle is in direct union with the ectoderm lying over it 

 (fig. 9, page 40), so that there is no space between the mantle and the test, but in 

 specimens preserved in alcohol the mantle contracts away from the test and leaves a 

 large cavity between, the only points of union being the sides of the branchial and 

 atrial siphons, and the place near the posterior end of the body where the large blood- 

 vessels pass across from the mantle to the test. This separation takes place much 

 more readily in some species than in others, while in some (Pelonaia corrugata, Poly- 

 carpa viridis, &c.) it apparently never occurs. 



The muscular fibres of the mantle are in all cases unstriped. They are either fusiform, 

 or very long filiform fibres, usually more or less united together into bundles. Round 

 the two siphons they are arranged so as to form strong sphincters, sometimes of consider- 

 able size (fig. 3, sph. page 32). 



In the Molgulidge the mantle is usually thin and membranous, and the muscular 

 bundles have a most characteristic appearance. They are united into short fusiform 

 packets, which present a striking resemblance to the typical form of a muscle in the 

 higher animals ; each having a broad central portion or belly, and two long tapering 

 tendon-like extremities. Another characteristic feature of the mantle in the Molgulidfe 

 is the arrangement of a large number of the bundles in lines radiating from the lower 

 edges of the sphincters so as to form a stellate figure round the base of each siphon. 



In the Cynthiidae the mantle attains its greatest thickness, and in some cases the 

 muscular system is developed to an enormous extent. The sphincters are always strong, 

 and the musculature is usually equally developed in the whole mantle, except in some 

 cases in the region over the intestine. The bundles are, as a rule, placed regularly and 

 parallel, forming longitudinal and circular coats. In most typical Cyntkiidaa there is 

 an internal circular covered by an external longitudinal coat, and in some {e.g., Poly- 

 carpa varians and Styela canopus) a third internal longitudinal layer is added. In some 

 Bolteninse the muscular system, though arranged upon the same plan, is so reduced in 

 amount that continuous coats are not formed, and the longitudinal and circular bundles 

 form a network with large rectangular meshes. 



