UnOCHORDA. 19 



invagination of a disc-like thickening of the epidermis in front of the 

 nervous system on the dorsal side of the body; and the atrial cavity arises 

 behind the nervous system by a similar process at a slightly later period. 

 Tlie gill clefts opening into the atrial cavity are formed as in the type of 

 simple Ascidians described by Krohn. 



The embiyo becomes hatched not long after the formation of the oral 

 and atrial openings, and the five epiblastic processes undergo atrophy. 

 They are not employed in the attachment of the adult. 



The larva when hatched agrees in most important points with the adult ; 

 and is without the characteristic provisional larval organs of ordinary 

 forms; neither organs of special sense nor a tail becoming developed. It has 

 been suggested by Kupfier that the ventrally situated mesoblastic mass is 

 the same structure as the mass of elements which results in ordinary types 

 from the degeneration of the tail. If this suggestictfi is true it is difiicult 

 to believe that this mass has any other than a nutritive function. 



The larva of Ascidia ampulloides described by P. van Beneden is 

 regarded by KupiFer as intermediate between the Molgula larva and the 

 normal type, in that the larval tail and notochord and a pigment spot are 

 first developed, while after the atrophy of these organs peculiar processes 

 like those of Molgula make their appearance. 



Sedentaria. The develoj)ment of the fixed composite Ascidians is, so 

 far as we know, in the main similar to that of the simple Ascidians. 

 The larvsB of Botryllus sometimes attain, while still in the free state, a 

 higher stage of development with reference to the number of gill slits, etc. 

 than that reached by the simple Ascidians, and in some instances (Botryllus 

 auratus Metschnikoff) eight conical processes are found springing in a ring- 

 like fashion around the trunk. The presence of these processes has led 

 to somewhat remarkable views about the morphology of the group ; in that 

 they were regarded by Kolliker, Sai-s, etc. as separate individuals, and it 

 was supposed that the product of each ovum was not a single individual, 

 but a whole system of individuals with a common cloaca. 



The researches of Metschnikoff (No. 32), Krohn (No. 25), and Giard 

 (No. 12), etc. demonstrate that this paradoxical view is untenable, and 

 that each ovum only gives rise to a single embryo, while the stellate systems 

 are subsequently formed by budding. 



Natantia. Our knowledge of the development of Pyrosoma is 

 mainly due to Huxley (No. 16) and Kovvalevsky (No. 22), In 

 each individual of a colony of Pyrosoma only a single egg comes to 

 maturity at one time. This egg is contained in a capsule formed of 

 a structureless wall lined by a flattened epithelioid layer. From this 

 capsule a duct passes to the atrial cavity, which, though called the 

 oviduct, functions as an afferent duct for the spermatozoa. 



The segmentation is meroblastic, and the germinal disc adjoins 

 the opening of the oviduct. The segmentation is very similar to that 

 which occurs in Teleostei, and at its close the germinal disc has the 

 form of a cap of cells, without a trace of stratification or of a seg- 

 mentation cavity, resting upon the surface of the yolk, which forms 

 the main mass of the ovum. 



After segmentation the blastoderm, as v^e may call the layer of 



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