XIII 



PHYLUM CHORDATA 



continuous behind with the ventral vessel. The dorsal sinus, 

 having no definite walls, is not contractile; but a closed sac, 

 the cardiac sac (card, s.), situated on the dorsal side of the sinus, 

 has a muscular ventral wall, by the contractions of which the 

 blood may be propelled. 



The nervous system consists of dorsal and ventral strands 

 (dors, n., vent, n.) which extend throughout the length of the body. 

 These are merely thickenings of a layer of nerve-fibres which 

 extends over the entire body in the deeper part of the epidermis. 

 Here and there are giant nerve-cells. The part of the dorsal 

 strand which lies in the collar (collar-cord) is detached from 

 the epidermis ; it contains a larger number of the giant nerve-cells 

 than the rest ; in some species it contains a canal, the neuroccele, 

 opening in front and behind ; in others a closed canal ; in most 

 a number of separate cavities. At the posterior extremity 

 of the collar the dorsal and ventral strands are connected by 

 a ring-like thickening, and there is a thickening also round 

 the neck of the proboscis. There are no organs of special sense ; 

 but some cells of the epidermis on certain parts of the proboscis 

 and on the anterior edge of the collar seem to be of the character 

 of sensory cells. 



Reproductive Organs. The sexes are separate, and often 

 differ in colour ; the ovaries and testes are simple or branched 

 saccular organs arranged in a double row along the branchial 

 region of the trunk 

 and further back ; 

 they open on the 

 exterior by a series 

 of pores. 



The course of the 

 development (Fig. 

 708-710) differs in 

 different species. In 

 some it is compar- 

 atively direct ; in 

 others there is a 

 metamorphosis. Im- 

 pregnation is ex- 

 ternal. Segmenta- 

 tion is complete and 

 fairly regular, re- 

 sulting in the for- 

 mation of a bias- 

 tula, which is at first rounded, then flattened. On one side of 

 the flattened blastula an invagination takes place. The embryo 

 at this stage is covered with short cilia, with a ring of stronger 

 cilia. The aperture of invagination closes and the ectoderm 



FIG. 708. Development of Balanoglossus. A, stage of the 

 formation of the first grove (gr.). S, stage in which the 

 second groove has appeared, and the first gill-slit has become 

 developed ; co. collar ; <j. si. gill-slit ; pr. proboscis. (After 

 Bateson.) 



