130 LECTURES ON MOLLUSCA. 



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

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



Sigillina is also histellate ; i. e. hoth the mouth and anal orifice are 

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

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

 Aplidia or Sea-figs have often been confounded with Alcyonium. 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 PYROSOMIDJE. 



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 tf incandescent cylinders of iron." Humboldt observed 

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

 fishes were made visible. 



Family SALPID.E. 



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 

 chains of compound animals. That they were the same, was first dis- 

 covered by Chamisso, the author of the well-known " Man without a 

 Shadow." The solitary Salp always gives birth to the compound, 

 and those again to the single. Doliolum is intermediate between 

 Salpa and Pyrosoma. 



Family APPENDICULARIAD^I. {Larval Ascidians.) 

 The minute Appendicularias appear as cloudy patches of red color- 

 ing matter in the northern seas. They are little tadpole-shaped crea- 

 tures, and resemble the larval stage of the higher tribes of Tunicaries, 

 arrested at the first period of growth. 



CLASS POLYZOA. 



Among the creatures generally grouped together as zoophytes, and 

 forming the structures usually known as " Corallines," " Sea-weeds," 

 &c., are many which are found to have a much more complex organi- 

 zation than the rest. There is an excurrent opening distinct from the 

 inhalent cavity : and though their general habit of life resembles the 



