VOL. III.] Balaiioglossiis. 197 



The mesoblastic pouches have arisen as two pah'ed evaginations 

 from the lateral walls of the digestive tract. These four pouches be- 

 come entirely severed from their original connection, and form large 

 thin walled, entirely closed bags. They become so large in fact that 

 each pair almost entirely surrounds the digestive tube, their inner 

 walls being in contact with this latter while their outer walls are in 

 contact with the inner surface of the ectoderm. In short, they form 

 the real body cavity, or coelom. The heart is a peculiar structure. 

 It is said to arise as a space, merely, between the water vesicle and 

 the proboscis gland. The walls of these two latter organs become 

 closely pressed against each other, the contact being interrupted in 

 a small area only, and this is the heart which becomes filled with a 

 fluid in which there are no cellular elements. This makes the walls 

 of the heart to consist of parts of the walls of two other organs, 

 and this means that if each of these has a function of its 

 own the tissue of the heart has a triple office, viz.: the portion 

 forming a part of the wall of the water vesicle functions in that 

 capacity; the portion belonging to the proboscis gland performs its 

 office there, and finally the two parts together perform the functions 

 of a heart. The organ can be very distinctly seen in the living larva 

 when placed on its side and flattened down somewhat with a com- 

 pressor (Fig. 4 is drawn from such a preparation). The walls are 

 very distinct, and the contractions constant and regular. It should 

 be pointed out, however, that the contractions are of quite a differ- 

 ent character from what is seen in the hearts of most other animals. 

 It does not consist either in a uniform, simultaneous contraction of 

 the entire wall, as one sees take place, for instance, in the spherical 

 vascular organs on the sides of certain marine leeches; or of a wave 

 of contraction passing from one end to the other, the contraction 

 affecting the entire circumference at each successive point passed 

 over by the wave, as takes place in peristaltic movement, 

 or as is seen in the heart of Ascidians, for example. But one- 

 half of the wall does not contract at all, while in the other half 

 a sharp fold sink deep into the cavity of the organ and travels 

 across it, the edge of the fold not extending across, however, to 

 the opposite wall. 



The gills have arisen as paired pouches from the dorsal wall of 

 the oesophagus, the anterior pair appearing first and the others in 

 succession behind them. They do not fuse with the ectoderm and 



