10 



SALPIDiE. 



trace of the passage of the small chain. When the solitary Salpcs 

 are kept in confinement for any length of time, nothing is more 

 common than to find floating about diminutive Sa/pcs-chsdns, nearly 

 identical in every respect, except size, with the larger chains found 

 at the same time in the sea. These small chains usually consist of 

 from twenty to twenty-eight pairs ; they increase rapidly in size, as 

 we find them of all sizes during every month in which Salprs have 

 been noticed, from the chains just escaped to the largest, which have 

 already lost their solitary embryo. The mouth is placed beneath 

 the heart, at the upper extremity of the posterior part of the gill ; 

 it opens into a kind of oesophagus, and in the winding course of the 

 digestive cavity can readily be followed in specimens which have 

 lost the chain of Salpcs; the anus opens close behind the mouth in 

 the respiratory cavity. The ]>yriform tubes first noticed by Hux- 

 ley are readily seen in the solitary specimens, though they are more 

 plainly observed, as well as the eleoblast, in the aggregate form, 

 just after their csc^ape from the solitary Salpcc. 



The principal difference between the solitary and aggregate forms 

 is one of outline, and in the proportion of the difierent organs, which 



Fig. 353. 



(Fig. 4.) 



r. h. r. 



are essentially the same, except the organs of reproduction. The 

 individuals of the chains are all alike on one side ; that is, we find 

 the endostyle either slightly to the right or to the left of the median 

 line, according as the individuals are on the right or left row of the 

 chain. When seen from a1)0ve or below, the aggregate form has 

 not the regular barrel-shape so characteristic of the solitary Salpa; 

 it is more spindle-shaped, with two somewhat ill-defined conical 

 projections at the posterior extremity, into one of which the nucleus 

 projects, and into the other a spur of the posterior cavity coming 

 close to the surface, one of the eight spurs by which the respiratory 

 cavities of adjoining individuals are connected. Each individual is 



