ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 789 



three groups contain (1) the Coelenterata with undifferentiated blasto- 

 pore, the Polyzoa with the blastopore differentiated into mouth and 

 anus, and the Brachiopoda derived from a modified Polyzoon; 

 (2) in Annelids and Molluscs the circumblastoporal ring is some- 

 times indicated by tentacles ; in Nemerteans, Balanoglossus, and 

 Echinoderms the larvjB are more highly modified. 



Cardiac Rhythm of Invertebrates.* — Mr. W. B. Ransom thus 

 sums up the chief results of his investigations on this subject. 



In the hearts of the Cephalopoda and Gasteropoda which he 

 examined, as well as in those of the Tunicata, the muscular fibres are 

 transversely striated. 



This transverse striation has been noticed by other observers in 

 Lamellibranchiata and in other Gasteropoda, and may probably be 

 considered as the general rule in Mollusca. (It is interesting to 

 notice that in this gi'oup striation appears to begin in the heart, a 

 non-voluntary muscle ; as the other cases of its occurrence — muscles 

 of the buccal mass, retractor oculi, shell muscle of Lamellibranchiata 

 — do not appear to bo so numerous or so constant.) 



In no moUuscan or tunicate heart examined are ganglion cells to 

 be found. These are, however, occasionally simulated by plasma- 

 cells. 



In Octopus a system of nerves and ganglia supplies the heart, 

 connecting it with the respiratory centre, and forming a co-ordinating 

 mechanism between the systemic and branchial hearts. 



In all the animals examined the cardiac muscle has the power of 

 rhythmical contraction independently of nerve-structures. 



In Mollusca, the dependence of the ventricle on the auricle is far 

 less close than is the case in Vertebrata. The two organs are 

 physiologically isolated. 



The chief requisite for a regular rhythm is not the connection of 

 ventricle and auricle, but a sufficient internal tension. 



The two visceral nerves (" vagi ") supplying the heart of Octopus, 

 act as inhibitory nerves on the ventricle and auricle ; and a single 

 nerve with a similar function supplies the heart of Helix. 



In Aphjsia a nerve was traceable to the region of the auricle, but 

 its function was not ascertained. 



In Ptcrotrachea no nerves could bo found in the heart, and no 

 inhibition could be caused. The same was true for Tunicata. 



The fibres of the inhibitory nerve in Octopus and Helix appear to 

 bo uniform in function, i. e. not divisible into an accelerating and an 

 inhibiting set. 



The action of these fibres appears to consist in changing the 

 molecular processes resulting in rliythmical contraction into others 

 which prevent at the moment such contraction, but which result iu 

 increased irritability and power of rhythmical contraction. This 

 action may bo roughly expressed by saying that the nerve diverts the 

 muscle from expenditure to accumulation of contractile material. 



Inhibition of a heart by direct stimulation with a weak interrupted 



* Journ. of PlijHiol., v. ri885) pp. 2G1-'M1 (4 plf.). 

 8or. 2— Vol,. V- 3 F 



