REPORT ON THE TUNICATA. 
379 
water, and that they rapidly decrease in numbers as greater and greater depths are 
reached. They are not so well represented in the abyssal zone as the Simple Ascidians 
are, and, so far as is known, they do not extend to as great a depth. Twenty species 
of Si Tin pie Ascidians were found in the abyssal zone, and seven ^ species occurred at depths 
between 2000 and 3000 fathoms. 
Just as in the case of the Ascidise Simplices, it does not seem possible to establish any 
relation between the occurrence of Compound Ascidians and the nature of the bottom. 
The shallow water forms are chiefly found on rocky and stony shores, and in many cases 
are attached to Zoophytes, Sponges, and Algae. Those from deep water are found living 
on various kinds of deposit, including “ volcanic mud,” “ green sand,” “ Globigerina ooze,” 
“blue mud,” “Diatom ooze,” “rock,” &c. 
The temperature of the water appears also to have little influence upon the distribution 
of the Ascidiae Compositae, as the bottom temperatures at their localities show a consider- 
able range, extending from a little above freezing point (34”2° F.) upwards to 71° F, 
1 To these numhers must be added the additional Simple Ascidians described i Appendix A , below. 
