ALTERNATION OF GENERATIONS IN ANCHINIA. 



481 



young buds from which the} are derived are found scattered between the 

 developed zooids on the colonial tube. 



BABROIS compares the zooids of type I. to the lateral buds of Doliolum, and 

 those of the second type to the phorozooids or foster forms of Doliolum. The 

 three different forms of the asexually produced generation are believed to 

 develop in succession in the colony of Anchinia, the one replacing the other. 

 On the youngest stolons, zooids of the first type develop ; later, when the 

 proliferating stolon proper breaks up into portions, only zooids of type II. 

 develop, and these finally are>eplaced by the sexually mature forms. 



The budding of Anchinm has been described most in detail by BABROIS 

 (No. 77) and is in many respects of great interest. It appears that the develop- 

 ment of the three types of buds takes place as a rule in a very uniform manner, 

 although considerable 

 variety prevails in the 

 time and manner of de- 

 velopment of the organs 

 into which we cannot 

 here enter further. In 

 Doliolum, the prolifer- 

 ating stolon is composed 

 of a number of longi- 

 tudinal strands and, 

 consequently, even the 

 youngest buds show the 

 separate rudiments of 

 the most important 

 organs, but the stolon 

 of Anchinia (Fig. 249 13) 

 is composed merely of 

 ectoderm and an inner 

 cell-mass called by 

 BARROIS entoderm. The 

 same structure is ex- 

 hibited by the very 

 small or youngest buds 

 in which an ectodermal 



layer and a central cell-mass can be distinguished. The latter becomes 

 differentiated, in a way as yet insufficiently known, into the nervous system, 

 the enteric canal (pharynx + intestine), the pericardial vesicle and (in 

 types II. and III.) into the rudiment of the genital organs which appears 

 very early. Certain features of the later development of type I. are of special 

 interest, as showing close resemblance to the manner of development of the 

 Ascidian embryo. The nervous system is found in the form of a tube running 

 along the whole dorsal side of the bud (Fig. 250 A, n). The anterior part 

 of this tube gives rise to the ganglion and the ciliated pit, while the posterior 

 part changes into an unpaired nerve-strand (Fig. 250 B) which runs back-, 

 ward and ends in a visceral ganglion. This nerve no doubt corresponds to 

 the strand observed by ULJANIN in Doliolum and called the branchial nerve. 

 It may be regarded as the homologue of the " cordon ganglionnaire visceral " 



II 



FIG. 249. A, part of a stolon of Anchinia ; B, part of 

 the proliferating stolon, more highly magnified, c, 

 colonial tube ; d, younger buds ; e, points of attachment 

 of older individuals ; m, cellulose mantle ; sf, pro- 

 liferating stolon. 



