XIII 



PHYLUM CHORDATA 



-fir 



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 

 .neurocoele, opening in front and behind ; in others a closed canal ; 

 in most a number of separate cavities. Between the collar and the 

 trunk the dorsal and ventral strands are connected by a ring- 

 like thickening. 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 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. 

 666) differs in dif- 

 ferent species. In 

 some it is com- 

 paratively direct ; 

 in others there is 

 a metamorphosis. 

 Impregnation is ex- 

 ternal. Segmenta- 

 tion is complete and 

 fairly regular ; re- 

 sulting in the for- 

 mation of a blastula 

 which is at first 

 rounded, then flat- 

 tened. 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 

 becomes closed up, and the ectoderm and endoderm become com- 

 pletely separate. The embryo becomes elongated and a transverse 

 groove (gr.) appears (A) : the mouth is formed by an invagination 

 in the position of the groove. The anus is developed in the 

 position formerly occupied by the blastopore. Before the mouth 

 appears there are formed two diverticula of the archenteron which 

 become completely separated off, their cavities subsequently 

 giving rise to the cavities of the proboscis and of the collar 

 and the body cavity of the trunk. By the appearance of a second 

 transverse groove (2?) the body of the embryo becomes divided 

 into three parts an anterior, a middle and a posterior these 



Fig. 666. Development of Balanoglossus. A, stage of the 

 formation of the first groove (gr.). B, stage in which the 

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

 developed ; co. collar ; g. si. gill slit ; pr. proboscis. (After 

 Bateson.) 



