48 THE VOYAGE OF H.M.S. CHALLENGER. 



constituted by the oesophagus, the stomach, and the first portion of the intestine, while 

 the second portion forms the anterior segment of the loop, lying close to and parallel 

 with the posterior. From the dorsal end of this second portion of the intestine a short 

 rectum usually runs anteriorly towards the atrial aperture. Sometimes the loop of the 

 intestine is so long that, after extending to the ventral edge of the body, it is turned 

 round anteriorly and then dorsally, so as to almost completely surround the left genital 

 mass (PL V. fig. 9), which lies anteriorly to the intestine. 



In the Ascidise Composite the stomach and intestine always project beyond the 

 branchial sac, but to very different degrees in different forms. In Botryllus they are 

 close to the branchial sac, and there is no distinct abdomen ; while in Amaroucium and a 

 number of allied genera the intestine projects for a long way behind the branchial sac, 

 and an abdomen as long as, or longer than, the thorax is formed. Botrylloides, Didem- 

 num, and some other genera, show a state of affairs intermediate between these extremes. 

 In all these forms the oesophagus commences near or at the posterior end of the branchial 

 sac at the dorsal side, and after running posteriorly for a short distance opens into the 

 stomach. The intestine emerging from the opposite end of this organ still runs posteri- 

 orly for a short distance, and then turns round towards the ventral side to run anteriorly. 

 It afterwards crosses the oesophagus, so as to become dorsal again in its last part, which 

 opens as usual into the peribranchial cavity. 



In Pyrosoma, the alimentary canal is placed transversely at the posterior end of the 

 branchial sac, but on account of the position of the atrial aperture at the opposite 

 extremity of the body from the branchial, the intestine bends round posteriorly so as to 

 describe a circle. 



In the Thaliacea the digestive canal and genital glands are usually aggregated into a 

 small mass, the " nucleus," placed in the ventral region of the posterior end of the body. 



In Appendicularia the oesophagus commences at the narrow posterior end of the 

 branchial sac, and leads ventrally towards the large stomach, which lies transversely and 

 has two lobes, a right and a left. The latter gives rise to the intestine, which turns 

 forwards as a short rectum and terminates in an anus placed on the exterior of the body 

 about the middle of the ventral surface. 



In many Tunicates the wall of the intestine is raised up to form a fold, which projects 

 into the lumen (fig. 9, page 40, and fig. 12, ty., page 47). This "typhlosole" varies 

 somewhat in its development in different species, extending throughout the entire intes- 

 tine in ^ome (Styela tuberosa), wdiile in others it is only present in a part of it. The 

 connective tissue underneath the endoderm and forming the centre of the fold usually 

 contains large blood-vessels. 



Several glandular appendages are found in connection with the alimentary canal in 

 the Tunicata, and are, like a number of nondescript organs in other groups of the 

 Invertebrata, usually referred to as the liver. 



