40 



PHYLUM TUNIC ATA (UROCHORDA). 



surface of a large specimen with his finger, and describes how 

 " in a few seconds his name came out in letters of fire." The 

 light is emitted by two groups of cells in the 

 region of the mouth. There is a single genus 

 Pyrosoma Peron. 



The zooids usually lie with their ventral surfaces 

 towards the closed end of the colony, but in P. 

 elegans they have the opposite position. There is 

 a buccal and atrial sphincter muscle and a few 

 delicate bands of muscular fibre in the body wall. 

 The test contains cellular elements and some fibrous 

 bands, and the zooids near the common cloacal 

 opening give off two tubular prolongations of the 

 dorsal body-wall containing muscular fibres, which 

 appear to have the function of acting on the dia- 

 phragm. 



The structure of the zooids is well shown in Fig. 

 33. The viscera are behind the pharynx, and the 

 anus opens into the posterior part of the atrial 

 cavity. The atrial cavity sends forwards two 

 diverticula, one on each side of the pharynx, but 

 these are not continuous with one another on the 

 dorsal side of the pharynx, as in other Ascidians. 

 The lateral walls of the pharynx consist of from 20 

 to 50 transverse bars, crossed by from 15 to 30 

 longitudinal bars. The stigmata are quadrangular, 

 and the longitudinal bars are supposed to be inter 



nal, fine longitudinal bars being absent, as in Culeolus, etc., among simple 

 Ascidians and in Pharyngodictyon among the Synascidians. It is doubtful 

 however if this interpretation is correct (see p. 12). 

 There is the 



N 



FIG. 32. Diagram of a 

 section through a Py- 

 rosoma colony (after 

 Delage and Herouard). 



1 buccal appendage ; 



2 common cloaca ; 



3 zooids ; 4 diaphragm ; 

 5 cloacal opening. 



usual row of 

 tent acle s of 

 which the ven- 

 tral median is 

 the longest 

 (Fig. 33). The 

 dorsal lamina 

 has the form of 

 1 2 langu ets 

 which show no 

 relation to the 

 transverse bars. 

 The intestine is 



Wh 



A 



Br 



T 



curved, there is FIQ. 33. A zooid of Pyrosoma (from Perrier). A exhalent aperture ; 



a pvloric eland Af anus ; Br pharynx ; C heart ; End endostyle ; A T nerve ganglion ; 



... mouth Ov ovary ; St stolon ; T testis ; Wb peripharyngeal ring, 

 rami lying on 



the inte s t i n e . 



The heart is placed a little behind the end of the endostyle (C). The 

 gonads are placed in the body wall on the ventral side behind the intes- 

 tine. The ovary contains one ovum which ripens before the testis. The 



