1875.] 195 [Brooks. 



Krohn, Vogt, Huxley, Leuckart and others, and I have been able to 

 add little to what is known upon the subject. 



The atrium of Salpa has been supposed to lack those lateral por- 

 tions which, in most Tunicates, lie upon the sides of the branchial 

 sac and are called the lateral atria; but at an early stage these seem 

 to be present, as well as the mid-atrium, but the cavities of the lat- 

 eral atria never become connected with that of the branchial sac by 

 the formation of branchial slits ; and at a very early period of devel- 

 opment the walls of each lateral atrium unite, thus obliterating the 

 cavity, and giving rise to a broad layer of tissue upon each side of 

 the body, between the branchial sac and the so-called " muscular 

 tunic," the "outer tunic " of Huxley. 1 Rows of transverse splits 

 soon appear in these layers, which thus become divided to form 

 the muscular bands, which latter subsequently become united to the 

 inner surface of the outer tunic. (Fig. VIII, m.) 



The sides of the mid-atrium become united at two points, one on 

 each side, with the posterior surface of the branchial sac, and as the 

 atrial and branchial tunics are free from each other between these 

 regions of union, a median longitudinal sinus is thus formed which is 

 the " gill " or " hypopharyngeal band." The central portions of the 

 two regions where the tunics are united, are soon absorbed, and a 

 single branchial slit is thus formed on each side of the " gill." 



The earliest stages in the formation of the atrial chamber were 

 not observed, but nothing was seen which seemed to indicate that it 

 is formed, as in most Tunicates, by tubular invaginations of the outer 

 wall of the embryo. 



The cavity of the oesophagus is a prolongation of that of the 

 branchial sac, and was in direct communication with this at the 

 mouth when first observed. The stomach is formed as a diverticulum 

 from the side of the oesophagus, and the cavities of the two were 

 connected at all the periods observed, but the cavity of the intestine 

 originates independently, and at first is closed at both ends; the par- 

 tition between it and the stomach disappears first ; that at the anal 

 or atrial end persists some time longer. 



The few facts which I have been able to add to what is known of 

 the development of the salpa chain relate, for the most part, to the 

 earliest stages in the development of this, which has always been 

 considered the sexual generation ; and seem to prove that the solitary 



1 This "outer tunic" must not be confounded with the "cellulose test" o 

 Huxley, which covers it. 



