SUMMARY AND GENERAL REMARKS. 



As I intend to reserve the discussion of any questions which affect the Tunicata as a class 

 till the conclusion of the second part of this Report, I have confined myself in the presenl 

 section to a brief summary of the chief additions made by the Challenger expedition to 

 our knowledge of the Simple Ascidians ; to a few remarks upon structural points of 

 novelty or interest, which are not sufficiently brought out in the systematic part ; and to a 

 discussion of the phylogenetic relations of the Ascidise Simplices, so far as our present 

 knowledge of the group will permit us to make such investigations. 



In the following pages the remarks upon the different species are arranged in the 

 order in which the genera occur in the preceding systematic part of the work, beginning 

 with the highest and working downwards. 



Among the Molgulidse the most interesting new forms are the two species of Ascopera. 

 Like so many of the species from deep water, they have the posterior end of the body 

 prolonged to form a peduncle by which the animal is attached. In this respect, and in 

 having no hairs upon the test, and no adhering sand grains, they differ from typical 

 Molgulids. These peculiarities are, however, found in one of the species referred to the 

 genus Molgula, namely, Molgula pedunculata. The two species of Ascopera differ in 

 all their internal organs as well as in external appearance. The branchial sacs are very 

 distinct. That of Ascopera pedunculata (PL II. fig. 5) is regular, and has rather the 

 appearance of the branchial sac of one of the Cynthiidse, on account of the tendency of 

 the stigmata to lie in transverse rows. In Ascopera gigantea, on the other hand, they 

 are always irregularly curved and placed (PI. II. fig. 1), and consequently the sac in this 

 species has more of the characteristically Molgulid appearance. 



Molgula pedunculata shows affinities with Ascopera, not only in having a short 

 peduncle, and in the absence of adhering sand, but also in the structure of the branchial 

 sac, which has the stigmata in some places very slightly curved, and arranged in trans- 

 verse rows very much as in Ascopera pedunculata (compare PL V. fig. 3, and PL II. 

 fig. 5). However, this Cynthiad arrangement is continued for short distances only. 



The only other branchial sac among the Molgulidse which requires special notice is 

 that of Molgula pyriformis (PL VI. fig. 2). The longitudinal folds are in a rudimentaxy 

 condition, exactly corresponding to that found in Styela oblonga and Styela glans, 

 among the Cynthiidse. The stigmatic portion of the sac does not enter into the 

 folds, which are represented merely by longitudinal tracts, along which the interna] 



(ZOOL. CHALL. EXP. — PART XVII. — 1882.) R 36 



