PLEUROBRACHIA. 



29 



and slowly from a state of contraction, or again the little ball, 

 hardly perceptible against the side of the body, drops suddenly 

 to the bottom of 



Pier "Xl 



the tank in which 

 the animal is float- 

 ing, and one thinks 

 for a moment, so 

 slight is the thread- 

 like attachment, 

 that it has actual- 

 ly fallen from the 

 body ; but watch a 

 little longer, and 

 all the filaments 

 spread out along 

 the side of the 

 tliread, it expands 

 to its full length 

 and breadth, and resumes all its graceful evolutions. 



One word of the internal structure of these animals, to explain 

 its relation to the external appendages. The mouth opens into a 

 wide digestive cavity (Figs. 27, 28), enclosed between two verti- 

 cal tubes. Toward the opposite end of the body these tubes 

 terminate or unite in a single funnel-like canal, which is a reser- 

 voir as it were for the circulating fluid poured into it through an 

 opening in the bottom of the digestive cavity. The food in the 

 digestive cavity becomes liquefied^ by mingling with the water 

 entering with it at the mouth, and, thus prepared, it passes into 

 this canal, from which, as we shall presently see, all the circulat- 

 ing tubes ramif3dng throughout the body are fed. Two of these 

 circulating tubes, or, as they are called from the nature of the 

 liquid they contain, chymiferous tubes, are very large, starting 

 horizontally and at right angles with the digestive cavity from 

 the point of junction between the vertical tubes (Fig. 30) and 

 the canal. Presently they give off" two branches, these again 

 ramifying in two directions as they approach the peripliery, so 

 that each one of the first main tubes has multiplied to four, 



Fig. 29. Natural attitude of Pleurobrachia when in motion. 



