ritter: octacnemus. 



239 



test wall. T therefore conclude that the ectoderm has undergone such 

 extreme modification, the secretion of test having been completed, that 

 it is no longer recognizable by the methods of examination employed. 

 This disappearance of the ectodermal layer in adult tunicates would 

 appear to be no unusual thing. I recall especially my inability to 

 demonstrate the presence of the layer in the root-hairs, or tubes of 

 Bhizomolgula (Ritter, :01), where it must certainly have existed at an 

 early period in the life of the individual. It would seem that in many, 

 perhaps most, cases there is no addition to, nor renewal of, the test in 

 tunicates after it has once been fully formed, at least as far as the ecto- 

 derm, the original source of the cellulose matrix, is concerned. Whether 

 the test cells, derivatives of the mesoderm, take up this office and replace 

 the ectoderm in it is an interesting question on which we have, so far as 

 I am aware, no positive information. 



In order to make my interpretation square with certain facts observed 

 not only by Moseley and Herdman, but as well by myself, and with 

 certain other statements and conjectures by my predecessors, a brief 

 consideration of the points involved is necessary. 



In the first place, all our observations agree in finding the branchial 

 orifice to open directly into the dorsal chamber, as the diagrammatic 

 sectional figure of Herdman shows (see Fig. 1). Of course, if my 



■■■aim. 



Fig. 1. — Copy of Herdman's text Fig. 11, "Challenger" Reports, Vol. 27, p. 93. 

 For the letterings of Fig. 1 and Fig. 2, see p. 251. 



interpretation is correct, this cannot be so in life, as reference to text 

 Figure 2 makes obvious. It follows then that the cavity would liave to 

 be regarded as due to rupture. This rupture is probably caused by the 

 contraction, at death, of the strong muscle bands at m. b." Figures 1 and 

 3, the position of which is also indicated in text Figure 2. As the layer 

 in which these fibres are situated (the homology of which will be seen 



