TYPE PBOTOOHOBDATA. 621 



with ectoderm, which almost completely surrounds the body, 

 being wanting only along the ventral wall and at the extreme 

 anterior and posterior ends. This cavity is the atrium' and 

 its walls are termed the mantle. It is comparable to the 

 atrial cavity of AmpMoxus, but has a somewhat more exten- 

 sive development and arises in the larva as a pair of dorsally- 

 situated invaginations of the body-wall which, gradually in- 

 creasing in size, enclose the greater portion of the body, and 

 their openings, gradually approximating, finally fuse to form 

 the atrial aperture (Fig. 283, A), the dorsal partition between 

 them at the same time disappearing, so that the two cavities 

 become continuous. 



The external surface of both the mantle and the body 

 proper is covered with ectoderm, and rests below upon a 

 layer of mesodermal connective tissue which contains muscle- 

 fibres. The coelomic cavity consists of a number of lacunar 

 spaces which have, especially in the walls of the branchial 

 region, a more or less definite arrangement and serve as blood- 

 vessels. In a somewhat distinct space near the hind end of 

 the body is situated a tubular heart, whose walls are formed 

 of a single layer of cells, the inner ends of which are con- 

 verted into muscle-fibres. The contractions of the heart are 

 wavelike, starting from one end and passing gradually though 

 rather quickly towards the other ; but a remarkable peculiarity 

 is that after a certain number of beats, at each of which the 

 contraction-wave begins at one end, its course is reversed, 

 and for a similar number of beats it begins at the other end. 

 This change takes place with a certain amount of rhythm, 

 and at each change the course of the blood through a portion 

 at least of the body is reversed. At each end the heart opens 

 into a large lacuna, one of which runs along the ventral mid- 

 line of the branchial sac, while the other, giving ofl^ lacunar 

 branches to the intestine and test, runs along the dorsal mid- 

 line of the same region, smaller lacunae, traversing the bran- 

 chial bars, uniting the two vessels. The blood-plasma is 

 colorless and contains amoeboid corpuscles which are also 

 usually colorless, though a few colored ones, resembling the 

 pigmented cells of the test, are frequently found. 



The mouth opens into a capacious pharyngeal or branchial 



