i9o8.] EXCRETORY ORGANS OF METAZOA. 603 



2. Evaginated vesicles, open to the blastocoel. Here are to be 

 reckoned the outer nephridia of prosobranch and pulmonate mol- 

 luscan embryos, and probably the anal kidneys of opisthobranchs. 

 The latter have a method of formation similar to that of the others, 

 but they differ in position. 



3. Tubular invaginations terminating blindly in flame cells, with 

 the cavities of at least the capillaries intracellular. Their origin 

 from the ectoblast has been proved only in the case of the Nemertini 

 and Acanthocephala and with some doubt in the Polycladidea. Here 

 are to be placed the definitive nephridia of the Platodes, Nemertini, 

 Gastrotricha, Rotatoria, RJwdope, Acanthocephala, and the larval 

 nephridia of Phoronis; probably those of the Endoprocta should be 

 placed here (if they are not mesectoblastic), and perhaps those of 

 the Priapulida and the head kidneys of some Alolluscan larvze. 

 This type of excretory organ has been named by Hatschek (1888) 

 protonephridium, though he extended this term to cover also organs 

 of mesectoblastic and even mesentoblastic origin. This is a very 

 natural group of excretory organs, showing great similarity in both 

 structure and development. The only case of a larval or head 

 kidney among them is that of Phoronis, yet here this kidney persists 

 into the adult though it later joins with a coelomostome. Kaiser 

 (1892) is inclined to compare the organs of the Acanthocephala 

 with those of Annelids or even with the anal kidneys of Bon cilia, 

 but their strictly ectoblastic origin renders this view unlikely; while 

 those of the Acanthocephala open into the genital ducts, so also do 

 those of certain Turbellaria, consequently this relation does not 

 speak against their community. 



4. Tubular invaginations with wholly intercellular cavity, with- 

 out flame cells of cilia. These are the Alalpighian vessels of 

 Insects and Chilopods (? and of other ]\Iyriopods), the proctodseal 

 organs of the Acarina, and possibly the rectal tubes of the Tardi- 

 grada. All of these either open into the proctodgeum or upon the 

 surface of the body near the anus; it is probable they secondarily 

 acquired the proctodseal position when the ectoblast invaginated to 

 produce the end-gut. These tubes are usually unbranched, but in 

 some Insects they are dendritic. They differ from type 3 mainly in 

 lacking cilia and in possessing a wholly intercellular cavity; but the 



