ASCIDIA AGGREGATA. 91 



is placed in the tube formed by the mantle towards its anterior 

 extremity ; and the heart, Hver, and other viscera, are united 

 into a small mass near this opening ; the anus is situate far be- 

 hind, and the only branchia, which is formed of a membrane 

 transversely plaited, extends obliquely from the superior to the 

 inferior parietes of the pallial cavity ; the water which traverses 

 this tube, consequently laves the respiratory apparatus, and the 

 animal moves by forcibly expelling it from the side of the mouth : 

 hence, we see it must necessarily swim backwards. When 

 full grown, these mollusks are free ; but at birth they are fre- 

 quently united to each other, forming a long chain, and swim 

 in this manner for a long time ; it seems that the individuals 

 thus united, after becoming free or separated, produce young, 

 which are not joined together in a chain as just described, and 

 differ from them in form, and thai the young arising from the 

 last are united and similar to the first ; so that in these singu- 

 lar animals there is the most remarkable alternation, the same 

 form and the same mode of existence not being transmitted from 

 one generation to the other, but constantly returning to the 

 second generation. 



. The Biphorse are found in the Mediterranean, and in the 

 warmer* regions of the ocean ; they frequently emit a phosphoric 

 light. 



28. The simple Ascidi^ cannot move like the Biphorae, but 

 live attached to rocks ; their mantle is in the form of a sack, 

 with two orifices, and the interior of this cavity is lined with a 

 net-work composed of the branchial vessels; the mouth and the 

 little bag or sack containing the viscera, are attached to the bot- 

 tom of the great branchial cavity, and the anus is found near one 

 of its openings. 



29. Other Tunicata, closely resembling the preceding in their 

 organization, live united in a common mass, and for this reason 

 they are designated under the name of Ascidia composita or 

 Aggregata. a gelatinous or cartilaginous tissue encloses a 

 great number of these little beings, the surface of which presents 

 a multitude of six-pointed stars formed by their openings ; their 

 propagation seems to be effected in two ways ; sometimes the 

 mass grows by the development of reproductive buds in this 

 common tissue, sometimes the young formed in an ovary are 

 expelled externally, and swim about free or separately for some 

 time, until they become attached to some submarine body, where 

 they establish a new colony. 



28. What are the characters of the genus Ascidia ? (Ascidia, from the 

 Greek, asJcos, a bottle or pouch.) 



29. What is meant by the Aggregata? (Aggregata; Latin, gathered 

 together.) 



