96 SHELL GALLERY. 



The remarkable Rhopalcea neapoUtana, from Naples, may be 

 roughly compared to an hour-glass with a very long constriction. 

 The test is smooth in the upper part, but knobby and encrusted with 

 foreign bodies below. The upper or thoracic end contains the 

 branchial sac, and the lower or abdominal portion the stomach, 

 heart, and reproductive organs, the gullet and intestine traversing 

 the whole length of the narrow central region. Although from its 

 general structure Rlwpalma is a Olavelinid, it is not certainly known 

 to produce buds. 



Perophora listen (Fig. 15) occurs in the form of little jelly-like 

 transparent blobs rising by short stalks from a silvery thread-like 

 stolon. Owing to their small size and transparency, it is possible to 

 examine specimens alive under the microscope, the currents passing 

 through the stigmata in the walls of the branchial sac, and the 

 beating of the heart being distinctly visible. The rapid motion of 

 the cilia surrounding stigmata gives the appearance of dark wheels 

 all rotating in the same direction. The heart beats so as to drive 

 the blood current so many times in one direction, and then after a 

 short pause, in the reverse direction. 



The exhibited specimen growing on an oyster shell, is from 

 Plymouth. 



Suh-order 2. — Ascidi^ Composite. 



The Compound Ascidians are fixed forms, which give rise to 

 colonies by budding, the individuals being immersed in a common 

 mass and not possessing separate tests. 



Although reduced to an extremely small size each individual or 

 ascidiozooid of a colony possesses the same organs as a large Simple 

 Ascidian, excepting that the former does not possess a separate test. 

 Frequently the individuals of a colony are grouped into systems, in 

 which the atrial orifices open into a common cloaca. The little 

 ascidiozooids vary greatly in shape in the different families. In the 

 Polyclinidm, for instance, they are long, the organs being, so to speak, 

 drawn out, and being arranged in three regions, the thoracic, ab- 

 dominal and post-abdominal, the first region containing the branchial 

 sac, the second the stomach, and the third the heart and repro- 

 ductive organs. In the DistomidcB, the body exhibits two regions, 

 thoracic and abdominal, the heart and reproductive organs lying 

 alongside of the stomach. The BotryllidcB comprise only one region, 

 the stomach and the other organs being situated by the side of the 

 branchial sac. 



