28 MARINE ANIMALS OF MASSACHUSETTS BAY. 



this highest group of Acalephs to assume a bi- 

 '^' ^ ' lateral character. This bilaterality becomes still 



more marked in the highest class of Radiates, the 

 Echinoderms. Such structural tendencies in the 

 lower animals, hinting at laws to be more fully 

 developed in the higher forms, are always signifi 

 cant, as showing the intimate relation between all 

 parts of the plan of creation. This inequality of 

 the diameters is connected with the disposition of parts in the 

 whole structure, the locomotive fringes and the vertical tubes 

 connected with them being arranged in sets of four on either side 

 of a plane passing through the longer diameter, showing thus a 

 tendency toward the establishment of a right and left side of the 

 body, instead of the perfectly equal disposition of parts around a 

 common centre, as in the lower Radiates. 



The Pleurobrachia are so transparent, that, with some prepara- 

 tory explanation of their structure, the most unscientific observer 

 may trace the relation of parts in them. At one end of the sphere 

 is the transverse split (Fig. 27), that serves them as a mouth ; at 

 the opposite pole is a small circumscribed area, in the centre 

 of which is a dark eye-speck. The eight rows of locomotive 

 fringes run ft-om pole to pole, dividing the whole surface of the 

 body like the ribs on a melon. (Figs. 27, 28.) Hanging from 

 either side of the body, a little above the area in which the eye- 

 speck is placed, are two most extraordinary appendages in the 

 shape of long tentacles, possessing such wonderful power of ex 

 tension and contraction that, while at one moment they may be 

 knotted into a little compact mass no bigger than a pin's head, 

 drawn up close against the side of the body, or hidden within it, 

 the next instant they may be floating behind it in various posi- 

 tions to a distance of half a yard and more, putting out at the 

 same time soft plumy fringes (Fig. 29) along one side, like the 

 beard of a feather. One who has never seen these animals may 

 well be pardoned for doubting even the most literal and matter- 

 of-fkct account of these singular tentacles. There is no variety 

 of curve or spiral that does not seem to be represented in their 

 evolutions. Sometimes they unfold gradually, creeping out softly 



Fig. 28. Pleurobrachia seen in plane of tentacles. {Agassiz.) 



