32 Transactions of the Royal Canadian Institute 



anterior plate in place of the usual number (two). That the division of 

 the disk into certain definite plates is a very fundamental feature in the 

 structure of these animals is shown by the constancy in the relation of 

 the muscles to the grooves separating the plates. Each groove has 

 usually along its entire length a series of short m.uscular bands, which 

 connect the adjacent plates and hold them together in the contracted 

 condition of the animal. 



It is surprising that there should be a lack of correspondence between 

 the siphonal and the extrasiphonal plates both in number and in arrange- 

 ment. In spite of this a certain order is apparent. The four extrasiphonal 

 plates are symmetrically arranged, there being an anterior, a posterior, a 

 right and a left. The condition of the lobation of the apertures in certain 

 other Ascidians indicates a primitive division of the apertural margin 

 into anterior and posterior halves. Each half may be subdivided into 

 right and left halves. Such a method of division of the apertural margin 

 characterizes the families, Botryllidae, Styelidae, and Tethyidae {Cyn- 

 thiidae) . 



In the atrial extrasiphonal region of Chelyosoma madeayanum (See 

 PI. I, fig. i) there can be no doubt that the grooves are diagonal and 

 that two of the plates are median in position, in this way being the 

 reverse of the condition in the Stylelidae, etc. The oral extrasiphonal 

 plates are the same, except that the anterior plate has been divided into 

 two, although occasionally undivided, giving a median groove. The 

 atrial siphonal plates are six in number and are referable to those in the 

 extrasiphonal region by considering that anterior and posterior plates 

 have been subdivided, the lateral one remaining undivided. The homo- 

 logies of the oral siphonal plates are not so apparent. There is con- 

 siderable variation in the position of the plates with reference to a 

 median line passing through the two apertures but usually such a line 

 nearly bisects an anterior and a posterior plate, which should be con- 

 sidered as median in position. Such a condition may be derived from 

 the simple one of the atrial extrasiphonal series, by a division of each 

 lateral plate into two. This would mean that both apertures are pri- 

 marily four-lobed, the lobes being separated by diagonal clefts, and that 

 the six-lobed condition is attained at the oral aperture by a division of 

 the lateral lobes, and at the atrial by a division of the median. The 

 plates, therefore, are related to the apertures and may be considered as 

 formed by a series of divisions, the first two being diagonal, and the later 

 ones median and transverse (See text fig. i). 



While the siphonal areas are quite definitely circular, the disk departs 

 considerably in shape from an ellipse, being broader behind than in 

 front, and the left side being usually larger than the right. The ex- 



