THE ANATOMY OF BALANUS. 97 



washed away. It is interesting to note that among the sessile 

 Cirripedes, the Balanidse, &c., these mantle folds are greatly 

 developed, and in some cases have their surface area increased by 

 further folding. Very probably their function under such con- 

 ditions is branchial, and we may be warranted in describing them as 

 branchiae. 



As in all other Cirripedes, no distinct blood system can be 

 traced in Lepas. 



The second common Cirripede type in our seas, is the little 

 Acorn-shell, or Rock-barnacle (Balanus). In larval history it is 

 identical with Lepas, but in external adult form, it presents a 

 wonderful contrast. Fleshy stalk has entirely disappeared and the 

 pallial sac, which lodges the body proper, is therefore sessile. Much 

 modification of the shelly plates of the mantle has taken place, the 

 parts being so arranged as to form a shelly palisade, wherein six 

 distinct pieces are to be traced, roofed in by two pairs of plates, one 

 pair representing the terga of Lepas, the other and larger pair, the 

 scuta. When the tide flows over these tiny creatures, a slit-like 

 opening between these opercular plates is revealed, through which 

 sweep in and out, with elegant motion, tiny feather cirri, the thoracic 

 feet (Fig. 3). With the receding tide, life-functions are partly 

 suspended, the cirri are retracted, and the scuta and terga are shut 

 down tightly, so as to retain some moisture till the tide returns. 

 Quite an appreciable noise is made in the tightening of the valves 

 when in this quiescent condition, and the low crackling murmur 

 heard when walking over rocks thickly coated with Barnacles, is very 

 familiar to me. Apparently the vibration of a heavy footstep is 

 perceptible to them, and lest it betoken danger, they take the 

 precaution of tightening their opercular valves, and in so doing form 

 some tiny water-bubbles, which in breaking give out sufficient sound 

 to be very noticeable when joined in by thousands of individuals. 



In the anatomy of the body, apart from the mantle, there is 

 practical similarity to that of Lepas ; the main differences are that 

 the cement gland is greatly increased, the ovigerous frcna developed 

 into two large folds functioning probably as branchiae, while two special 

 and strong muscles are developed at either end of the rampart-like shell 

 to control the closing of the scuta and terga (m 1 and m 2 , Fig. 5). 

 Yet another and more important difference is seen in the ventral 

 nerve ganglia being concentrated into a single large ganglionic mass. 



The third type of Cirripede which I select, is as utterly unlike 

 either of the preceding as it is possible to conceive, appearing simply 

 as an oval bag without calcareous plates or even sculpturing, attached 

 to the under surface of the abdomen of crabs. From this external 

 sac penetrate into the interior of the host long branching tubuli, 



