ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 269 



experiments lead him to give tho following explanation. The two centres 

 of stimuli are physiologically equivalent, but both exhibit periodicity, 

 the stimuli passing through a cycle of variations in intensity, not syn- 

 chronous in the two centres. The stimuli are capable of being trans- 

 mitted from muscle-cell to muscle-cell, but the cells periodically enter 

 into a "refractory" state when they refuse to respond to certain stimuli. 

 The period of diminished irritability in centre A coincides with the 

 period of diminished power of transmission of the related muscle-cells, 

 so that it is centre B which controls the contractions. But during 

 this resting period, centre A recovers as B becomes exhausted, and alter 

 a period of antagonism, corresponding to the pause, the action of the 

 heart is reversed. As this is regularly repeated, the action of the heart 

 can be represented thus: — B — t-A, pause, A — >-B, and so on. The im- 

 portance of the phenomenon is that it shows how a perfectly co-ordinated 

 mechanism may exist in muscle without the intervention of nervous 

 elements. 



Pigmentation in Tunicates.* — Antoine Pizon observes that much, if 

 not all of the pigment in Distaplia rosea, Botrylloides rubrum, Botryllus 

 violaceus, &c, results from the histolysis of the individuals which die 

 within the corm ; that the larvae, at first almost colourless, receive pigment 

 granules by migration from the maternal body; that the pigmentation 

 increases as more ascidiozoids die off; and that there is a slow elimi- 

 nation of pigment via the tunics, especially in Diplosomidaa and Dis- 

 taplia. 



Excretory Organs of Tunicates.f— Wilhelm Dahlgriiu briefly sum- 

 marises the results of a series of observations which he has made on this 

 subject. He regards as the most primitive condition that found in 

 Botryllus, Polycyclus, and Ciona intestinalis, where an excretory organ is 

 represented by a small number of unmodified mesenchyme cells, in whose 

 protoplasm the excretory products form dark granules. In Saljxi de- 

 rnocratica-mucronata and S. ruminata-fusiformis, quite similar conditions 

 occur, which the author regards as con6rmation of tne view that Salpa 

 has arisen from a primitive and not from a specialised sedentary stock.. 

 The next stage in the development of the excretory organ is illustrated in 

 the Ascidiinae, where the excretory cells form an envelope round ihe 

 gut. The cells are united to form closed vesicles, into whose lumina the 

 excretory products are shed, and these vesicles are imbedded in a con- 

 nective-tissue stroma which surrounds the gut. In the Cynthiadas the 

 excretory organ consists of a small number of sacs of considerable size, 

 which lie immediately beneath the body-epithelium. Each is surrounded 

 by a connective-tissue capsule, and the constituent cells secrete crystal- 

 line rods which are shed into the lumen of the sac, and ultimately form 

 an amorphous mass of concretions. The highest stage in development 

 is represented in the Molgulidae, where there is a large kidney-vesicle 

 with a double-layered wall. The outer wall is of connective-tissue, the 

 inner of glandular epithelium. These cells, as also to a less extent 

 those of the kidney in the Cynthiadae, showed the rod-like structure seen 

 in the protoplasm of the kidney-cells in Cephalopoda. M 



* Comptes Rendus, cxxxii. (1901) pp. 170 2. 

 t Zool. Anzeig., xxiv. (1901) pp. 149-51. 



