100 
SHELL GALLEEY. 
Specimens vary in size from a few inches to upwards of four feet 
in length, and, as the name of the genus implies,* they are brilliantly 
phosphorescent. Sometimes they occur in innumerable multitudes, 
Fig. 20. 
Section through wall of Pyrosoma, magnified, showing a single layer of 
ascidiozooids. 
br, branchial ; at, atrial orifice ; tp, process of the test ; br s, branchial sac. 
(Herdman : Tunicata, Encyclopaedia Britannica.) 
giving rise to a zone of greenish light extending for miles. Professor 
Moseley records that during the voyage, of the Challenger in the 
North Atlantic a huge specimen of Pyrosoma spinosum, four feet 
in length, was captured. On tracing his name on its body, the word 
came out in letters of fire. 
In Pyrosoma elegans (exhibited), from Naples, the ascidiozooids are 
arranged in verticils, and the mouth of the cylinder is surrounded 
by a movable diaphragm ; the outer end of each ascidiozooid is 
provided with a membranous spine. Six species of Pyrosoma are 
known. Pyrosoma atlanticum is found in the tropical Atlantic and 
Antarctic ; P. giganteum in the Atlantic, Pacific, and Antarctic ; and 
P. spinosum in the South Atlantic. 
Order II. — Thaliacea. 
The Thaliacea are free-swimming Tunicates, which exhibit 
alternation of generations in their life history. There are three 
families, Salpidm, Octacnemidce, and Boliolidce. 
Salpidm. — The Salpas are transparent barrel-shaped organisms 
* Pyrosoma — pur, fire ; soma, body. 
