TUNICATA. 401 



by a peribranchial chamber, which commwiicates with the ex- 

 terior by a special (atrial) opening. The heart is simple and 

 tubular, a?id there is a periodic reversal in the direction of the 

 blood current. Nephridia are absent, and the renal organs 

 are always devoid of ducts. All are hermaphrodite. There 

 is usually a metamorphosis in development. Colonies are fre- 

 qtiently formed. 



Though typically sedentary, the Tunicales show considerable variation 

 in habit. Many grow fixed to stones or shells, or to the muddy bottom, 

 and are common on or near the coasts of all seas. They live on minute 

 organisms carried into the pharynx by the water of respiration. Through- 

 out the group we see that antithesis between sessile vegetative forms and 

 active pelagic forms, which is so vividly exhibited in the Ccelentera. 

 Some of the free sw^imming forms are indeed as typically pelagic in 

 structure and habit as the medusfs themselves. Of the sessile forms 

 some are simple (e.g., Ascidia) ; others, in which the vegetative habit 

 has more throughly permeated the organism, reproduce themselves freely 

 by budding. The clusters so formed may consist of individuals united 

 only by a common blood system, forming the so-called social Ascidians 

 {e.g., Clavellina), or composite organisms may be formed as in Botryllus. 

 Again we have allied colonial forms, such as Pyrosoma, the phosphor- 

 escent fire flame, which are free swimming and pelagic, the whole colony, 

 with its numerous individuals, moving as one creature. All these 

 individuals are formed by budding from a rudimentary larva which arises 

 from the fertilised egg. 



All these types belong to the Ascidian series. Different from them, 

 but connected by Pyrosoma, are the free swimming genera Salpa and 

 Doliolinn, together with the strange abyssal, and probably sessile, 

 " Challenger " genus Octacnemus. Both Salpa and Doliolum exhibit a 

 complex alternation of generations, in the course of which both solitary 

 and aggregated forms occur, the latter, like the floating colonies of 

 Siphonophora, often showing considerable division of labour. 



Finally, there are a few active free swimming forms, which retain 

 many of the features of the larval Ascidian. Of these Appeiidicularia 

 is the simplest type. 



Type of TuNiCATA — a simple Ascidian {Ascidia mentula). 



In form, an adult Ascidia is an irregular oval of three to 

 four inches in length ; one end is attached to stones or 

 weed ; the other is more tapering, and terminates in the 

 mouth, close beside which, on the morphological dorsal 

 surface, lies another opening, the exhalent or atrial aperture. 

 During life, water is constantly being drawn in by the mouth, 

 and passed out by the atrial opening. If irritated, the 

 animal frequently drives a jet of water with considerable 

 force from this aperture, whence the name " sea squirt." 



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