ASCIDIA CEA THALIA CEA . 409 



The Larvacea form a group of great theoretical interest. As to their 

 phylogenetic position two views have been maintained. According to 

 one recently reasserted by Brooks, they are the slightly modified 

 descendants of the primitive Tunicates, from which the Ascidians have 

 diverged in the direction of degeneration ; in other words, they are 

 primitive forms. According to the other view, they have been derived 

 from an Ascidian-like form, in which degeneration had already taken 

 place, and may be compared to prematurely sexual larvae. 



Order 2. ASCIDIACEA. 



(a) Ascidioe simplices. Ascidia, Phallusia, Ciona. 



(b) Ascidke Composites. Botrylhis^ Polyclimim. 



(c) Pyrosomidce. Pyrosoma. 



The characters of this order may be gathered from the description of 

 Ascidia. Most are sedentary, with the notable exception of Pyrosoma. 

 In several there is an alternation of sexual and asexual forms in the life 

 history. In Pyrosoma the embryo develops within the body of the 

 mother ; there is much yolk, and development is shortened and direct. 

 The greater number of the Tunicates are included in this order, among 

 them several well known British genera. 



Order 3. THALIACEA. 



(a) Salpa, Octacnemus. 

 (b} Doliolum^ Anchinia. 



This order contains a few genera which show considerable modifica- 

 tion from the Ascidian type. With the possible exception of the little 

 known Octacnemus , all are free swimming, and, with the same excep- 

 tion, pelagic. The mouth is at one end of the body, the atrial aperture 

 at the other ; the animals swim by forcing the water out of the peri- 

 branchial chamber posteriorly. The cuticle is often very thin, as we 

 should expect in animals so active ; in some species it seems to be 

 absent. In Doliolum, the muscles form hoops encircling the barrel 

 shaped body ; in Salpa the body is fusiform, and the muscle bands do 

 not form complete rings. Both are devoid of reproductive ducts, and 

 very simple in structure. 



The life history shows an alternation of generations. In Salpa, a 

 solitary asexual form, or " nurse," gives rise to a complex stolon, which 

 segments to form a chain of embryos. This chain is set free, its 

 members become sexual, and, either while still united, or after separa- 

 tion, produce ova, which give rise to the nurse form. In Doliolum, a 

 small stolon forms a number of primitive buds, which creep over the 

 parent and multiply. They form a lateral series of individuals, which 

 furnish the parent form with food, and a median series which are set 

 free as asexual " foster mothers." These carry with them some 

 primitive buds, which divide into secondary buds, and these finally 

 become the solitary sexual forms, producing ova which develop into 

 " nurses." The " nurse " form during the budding process degenerates 

 greatly, until, like the swimming bells of the Siphonophora, it becomes 

 a mere organ of locomotion, nourished by the lateral buds. There is 

 thus considerable division of labour in the colonial form. 



