1910] Johnson: Quantitativt Study of Salpa Chain. 155 



We turn nexl to the orientation of the zooids relative to the 

 parent. When we compare the cross-sections with the sketch 

 showing the position of the whole chain, it would seem that the 

 genital rod is toward the dorsal side of the parent, a condition 

 the reverse of that described by Brooks. As this matter of position 

 lias no vital connection with the present problem, 1 have not 

 taken lime to make sure of this point so far as the first block is 

 concerned. It is certainly true that in the second bloek the 

 genital rod is toward the dorsal side of the parent, the zooids 

 standing on their heads. Frecpiently. as the chain grows larger, 

 this position is altered gradually until in the largest block the 

 zooids come to lie on their sides with their oral extremities turned 

 toward the nucleus of the parent. 



The blood tubes pointed out in the segmenting stolon persist 

 while tlie zooids change their shape in shifting from single to 

 double file. Their position in the distal part of the chain is 

 shown in fie-. :]. v. As the zooids grow, the vascular canal becomes 

 gradually smaller and the zooids by their own growth pull apart 

 from one another and the vascular connection is lost entirely. 

 The rotation of the zooid on its own axis after the two rows have 

 been formed, as described in the quotation from Brooks, involves 

 a structural change which is clearly shown in his sections and 

 diagrams but not so well in my sketches. 



In chains whose zooids are almost ready to fall apart, the 

 connection between zooids is by papillae. Quoting from Brooks 

 (Brooks. '93, p. 87): "The members of the community are 

 united to each other by processes from the walls of their bodies, 

 which are hollow and contain diverticula from their body cavi- 

 ties, although there is no communication between the body 

 cavities of adjacent salpae." These papillae grow gradually as 

 the vascular canal decreases in size and the zooids grow apart. 



The axis of the zooids in most blocks is at right angles to the 

 axis of the chain, but in blocks which are ready to leave the 

 parent or are found floating in the water, they become more and 

 more inclined. 



This gradual loss of connection with the parent and the 

 assumption of independent life by the zooids involves a change 

 in the source of food supply which may affect the rate of growth. 



