1866.] 21 [Agassiz, 



When seen from above 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 in direct 

 communication with no less than three adjoining ones, as will be seen 

 hereafter. When seen in profile, (fig. 4), the outline is ellipsoidal; 

 the two principal openings are placed at a distance from the extremi- 

 ties, the anterior spur of the tunic extending beyond the opening, 

 thus bringing both their openings rather more to the ventral side, and 

 not strictly along the continuation of the axis, but on each side of it. 

 There are only five muscular bands, one at the posterior extremity, 

 three others uniting on the ventral surface, somewhat behind the 

 anterior part of the gill, and another ill-defined one at the anterior 

 extremity. The nucleus (n.) is much larger in proportion to the body 

 than in the solitary form ; the endostyle (e.) occupies but a little more 

 than a third of the cavity. The gill, when seen in profile, runs some- 

 what obliquely towards the anterior extremity, where it is nearest the 

 ventral surface. There is no perceptible difference in the size of the 

 anterior and posterior openings of the respiratory cavity. The vibra- 

 tile cavity and the nervous ganglion do not differ in structure from 

 those of the solitary form ; the languet is perhaps somewhat broader and 

 more powerful. In the chains I have had the opportunity to examine, 

 I found either that the solitary fcctus had already been expelled, or 

 was only slightly developed, so that I can only say that its position 

 corresponds with what has been described by Sars, Krohn, Vogt and 

 others, the testes, as is well known, being greatly developed in the 

 individuals which had already lost their solitary foetus. What is 

 worthy of special notice in the aggregate form is the great thickness 

 of the tunic ; this would make the connection between the individu- 

 als of a chain simply a mechanical one, were it not for the spurs from 

 the respiratory cavity (s. c.) which project through the thickness and 

 connect with similar spurs in adjoining individuals. The spurs disap- 

 pear invariably after the individuals of a chain have become sepa- 

 rated for any length of time, and they are incapable of reuniting 

 again as has erroneously been asserted. When thus freed, the aggre- 

 gate form is perfectly helpless, the great thickness of the tunic pre- 

 venting it from regulating its motion ; while, when connected as a 

 chain, their capacity to guide the chain in any particular direction is 

 much greater. 



Sars has described exceedingly well the mode of aggregation of the 

 chain of Salpa runcinnata ; the chains, however, were quite far ad- 



