The Genus Synchceta. By C. F. Rousselet. 271 



cult to make out. After considerable trouble I have obtained a 

 number of isolated clean jaws and have mounted them separately. 

 There are two types of jaws in the Syncheetse which may be desig- 

 nated as the pectinata and tremula types, and which are represented 

 in fig. 7, pi. IV. and fig. 10, pi. V. The main difference between 

 these two types is that in the pectinata type the thin triangular 

 unci have no teeth, whilst in the tremula type they have six to 

 seven well developed teeth. In both types the fulcrum is very 

 long, thin in front view and wide or broad from the side, and the 

 narrow manubria, as well as the rami, have large rounded, but 

 extremely thin, wing-like lateral prolongations which are quite 

 invisible in the living animal, and can be seen only when the jaws 

 have been completely dissolved out with potash, and then only 

 satisfactorily by a good dark-ground illumination. When all the 

 parts are in position the jaws form a nearly globular structure, and 

 therefore no single view can give a good idea of the shape and form 

 of the parts. By transmitted light only the outline of these thin 

 plates is perceived. Fig. 7 shows a front view and fig. 7a a side 

 view of the jaws of S. pectinata, and fig. 10 a front view of those of 

 S. ohlonga, whilst fig. 10a represents the unci and fig. 105 a separated 

 manubrium of the same. The unci of S, tremula, triophthalma, 

 and vorax are shown in figs. 3&, 14a, and 196 respectively. The 

 jaws as a whole are nearly globular in shape, and therefore any 

 drawing of them, showing the various parts in position, is bound 

 to be semi-diagrammatic. The unci are situated immediately below 

 the shield-shaped mouth, ready to seize anything that may enter, 

 and a view of them with a high power can readily be obtained in 

 the living animal by adding one drop of 2 p.c. cocaine, or of 1 p.c. 

 eucaine, in a watch-glass full of water containing the Synchaata? 

 and then transferring some to a compressor, when the animals 

 will soon fix themselves with their heads to the cover-glass and 

 remain there for a long time almost motionless, except the move- 

 ment of cilia. With a high power (I use a fine Zeiss ^ apochro- 

 matic water-immersion) this affords an excellent view of the whole 

 front of the head, which it is otherwise almost impossible to 

 obtain. 



The great majority of Synchaetae have jaws of the tremula type. 

 I do not think any Synchaeta can project its jaws through the 

 mouth, for the unci are quite twice as large as the shield-shaped 

 mouth-opening. The heart-shaped mastax contains in its interior 

 a roughly dumb-bell shaped cavity. The muscles of the mastax 

 can expand this cavity suddenly, and it is my belief that by this 

 sucking action the food-particles, consisting of diatoms, algae, infu- 

 soria, and small rotifers, are forced into the mouth when they are 

 at once seized by the unci. 



Figs. 11a and 12a, pi. VI., represent the front view of the head 

 of S. baltica and S. monopus respectively, which have been drawn 



T 2 



