278 LECTURES ON MOLLUSCA. 



Family BoTRYLLiDiE. {Compound Ascidians.) 



These creatures have their tests fused into a common mass, so that 

 each zooid looks like a single animal outside ; but the individuals are 

 found to be separate within. In the Botryllians, the individuals are 

 united into systems round common excretary cavities. In the Didem- 

 nians, the chest and abdomen are distinct. In the Polyclinians, there 

 is a chest, with the breathing organs ; an upper abdomen, with the 

 digestive organs ; and a lower abdomen, with the heart (so called) and 

 reproductive organs. 



In Botryllus, the breathing-holes are star-shaped, the cloaca being 

 poured into a common sewer. In Botrylloides, the stars are more 

 irregular, and the animals are vertical. 



The zooid of Didemnium is very irregular, the individuals with a 

 pedunculate abdomen. In Euccelium, the animals are scattered, or 

 arranged in quincunx. Leptoclinum makes thin, variously colored, 

 zooids, adhering to the roots of tangles. Distomus and Diazona are 

 bistellate, the latter being flower-shaped, like Syntethys. 



Sigillina is also bistellate ; i. e. both the mouth and anal orifice are 

 rayed. The zooid grows like a plantain. In the remaining genera, 

 the mouth only is rayed. Polyclinum has a fungus-shaped mass. The 

 Aplidia or jSea-figs have often been confounded with Alcyoniwn. Sid- 

 nyum forms transparent, amber-colored masses under shelving rocks at 

 extreme low water. Syncecium is an arctic form, with a stalked zooid. 

 Amcercecium has a common central cloaca to the pod-shaped zooid. 



Family Pyrosomid^e. 



The Pyrosomes combine in innumerable numbers to form hollow 

 transparent tubes, open at one end, which receive the common cloaca. 

 These tubes, or zooids, are from two to fourteen inches long, and an 

 inch across. The mouths are outside ; and by the combined force of 

 the exhalent currents, the zooid is driven forward in the open sea 

 with the closed end forward, reminding us in a feeble manner of the 

 squirt-swimming of the Cuttles. They increase by buds or by eggs : 

 and often fill the sea in such vast numbers as greatly to incommode 

 the nets of fishermen. At night they are brilliantly phosphorescent, 

 resembling "incandescent cylinders of iron." Humboldt observed 

 them as forming lights, eighteen' inches in diameter, by which the 

 fishes were made visible. 



Family Salpid^;. 



The Salpas first exhibit to us the zoophitic condition of alternate 

 generation. No Salpa is like its parent or its child ; but always re- 

 sembles its grandparent or grandchild. The creatures of one genera- 

 tion therefore do not exhibit to us the whole Salpoid structure. Just 

 as in the higher animals we must have two individuals, male and 

 female, before we can gain a complete idea of the species ; so in the 

 Salpas we must see two generations, mother and child, before we can 

 understand the complete Salphine zooid. The Salpas are found under 

 two very contrary conditions ; as free individuals and as serpentine 



