PROTOSTIOMATA OF M()I,GULA MANHATTENSIS. 153 



I desire to lay special stress upon tlie protostigmata A, B, 

 0, and D. It is often difficult to distinguisli between an 

 actual observation and an interpretation of one. There is not 

 the slightest doubt in the world that the origin of the proto- 

 stigmata C and D from A and B in Molgula manhattensis 

 takes place in the manner described in the preceding lines. 



There are j^ei'liaps two main points of interest which are 

 brought out by comparing the protostigmata of M. manhat- 

 tensis \vith those of Ciona intestinalis. 'J'he first is a 

 question of homology, and the second is one of homodynamy. 

 The stigma D is the third protostigma in C. intestinalis and 

 the fourth in M. manhattensis. It arises from abstriction 

 from B in both cases, but in reversed senses. We have, there- 

 fore, here a special instance of enantiomorphism. 



The point of homodynamy relates to the two protostigmata 

 Avhich are the first to be formed, namely A and B. In my 

 judgment the behaviour of these clefts both in Ciona and 

 Molgula, and also in Phallusia, shows them clearly to be 

 liomodynamous formations. Protostigma A is oljviously homo- 

 dynamous with protostigma B. This is my contention, but it 

 is not the opinion of Professor Julin. 



In the work to which reference has already been made 

 (above, p. 148) i\I. Julin, in developing an idea previously 

 expressed by him in conjunction with Professor E. van Beneden 

 (1887), maintains that the first primary branchial stigma is 

 quite distinct in its origin and fate from the second, benig 

 essentially concerned, in its capacity of branchial pouch, with 

 the formation of the pcribranchial chamber. According to 

 Julin,^ the pcribranchial chamber is essentially formed by 

 dilatation of the walls of the hrst pair of primary branchial 

 stigmata, and the part taken by the atrial involutions is quite 

 subordinate, if not altoo-etlier neo-Hgible. 



^ It, should be stated lliat M. Juliii's observations relate principally to 

 Ciona intestinalis, Litlionepliria eugyranda, and Styelopsis 

 grossularia. In the last two species there is at no stage a paired atrium, 

 the cloacal aperture being unpaired and median from tlic beginning. I do 

 not attempt to contradict his observations on these species, but his interpre- 

 tation is open to discussion. 



