PHYSIOLOGY OF THE INVERTEBRATA. 239 



Anodonta, Unio, Octopus, Pahulma, Linmccits, Patella, TIclie, 

 Purpura, Arion, Limao:, and Littorina. 



The enterochloropliyll occurs dissolved in oil globules, also 

 in the granular form, and it is sometimes dissolved in the 

 protoplasm of the secreting cells of the so-called liver. In 

 hibernating snails, these pigments occur in greater abundance 

 in the winter than in the summer time. 



In concluding these remarks on the principal colouring 

 matters of the Mollusca, we may state that enterohrematin* 

 occurs in the " livers" of Helix, Limax, Arion, and Patella; 

 and MacMunn believes that enterohasmatin is probably the 

 mother-substance of those histoha-matins, which are found in 

 animals in whose " livers " it is built up. 



The Tunicata. 



In the Phari/ngopneustal Series {JEntcropneiistra and Tuni- 

 cata) a new form of internal aquatic respiratory organ 

 appears. The dilated pharynx of the Poh/zoa is farther 

 developed in these animals, being perforated by lateral open- 

 ings. These openings are ciliated, and a constant stream of 

 water enters at the mouth or oral aperture, which passes into 

 the pharynx through the openings (branchial clefts), then 

 into the atrial chamber (which is developed round the 

 pharynx), and out of its aperture into the air in which the 

 animal lives. As in the Pohjzoa, the walls of the pharynx 

 are well supplied Avith blood-vessels. 



The Tunicata have been called sea-squirts, for if they are 



irritated they suddenly contract the muscular walls of the 



body, and this contraction causes the water contained in tlie 



atrial and branchial cavities to squirt out in two jets. It may 



be remarked that the late Mr. Darwin considered the Tunicate 



as the representative of the point at which the Vertehrata 



began to work off from primordial forms common to it and to 



the Mollusca. 



* Enterohasmatin is synonymous with hajmochromogen and helico-rubin 

 of Krukenberg. 



