ACALEPHJE. 109 



tains cells which are full of air. Above, this disc is covered by the 

 integument alone; below, it sustains all the parts of which the 

 [compound] animal consists. 



[The shell of Velella with its horizontal and perpendicular plates 

 consists of a single piece. The thicker horizontal portion is formed 

 of two lamiiise connected by perpendicular concentric pieces, so that 

 annular canals are formed which are filled with air. These canals 

 communicate with each other in Velella, but not in Porpita : in both 

 they open externally by many minute pores on the upper surface. 

 The soft parts constitute a mantle which covers the shell and projects 

 beyond its edge by a free border. At the inferior excavated portion 

 of the shell, the mantle has on its outside the attached polyps and 

 appendages, on the inside the large liver. 



The polyps are of two sorts, a single large and central polyp, and 

 many small ones disposed around it in irregular rows. They have 

 been designated " stomachs " and " suctorial tubes." But observers 

 do not agree respecting their function. LESSON attributes to both sets 

 a digestive power, Voyage de la Goquille, pp. 49 56, and Acalephes 

 p. 561 : whilst Y. SIEBOLD Vergl. Anat. s. 63, note, thinks that the 

 smaller polyps alone discharge the office of digestion, and consigns 

 the large one to the respiratory system : and HOLLAED Ann. des Sc. 

 Nat. T. in. 1845, p. 250, says that the large central pouch is the 

 stomach and the small ones canaux aquiferes. KOELLIKER however 

 assures us that he has found small Crustacea both in the large and 

 the small tubes, and has seen the residue of digestion pass from 

 them all indifferently. Consequently we conclude with him and 

 others, that the Velellidce are colonies or compound animals. 



The liver is a large brownish mass placed above the central 

 stomach : it fills the inferior cavity of the horizontal plate. It is a 

 collection of fine canals formed of homogeneous membrane lined 

 with brown cells. A certain number of the canals branch from two 

 openings in the base of the central polyp : they frequently anastomose 

 and form a network on the surface of the liver from which fine 

 vessels pass to the perpendicular plate and to the margin of the 

 horizontal plate (Velella). These vessels, then, would seem to have 

 received the nutriment which has passed from the central stomach 

 into the liver-canals, for the purpose of redistribution to the soft 

 parts when it has been modified by the biliary secretion. Of the 

 smaller polyps a few, which hang beneath that part of the liver which 

 projects beyond the large polyp, open into liver-canals: but the 



