100 MARINE ANIMALS OF MASSACHUSETTS BAY. 



measures, when fully expanded, some fifteen to eighteen inches 

 in length. Unlike the Cuvieria, the ambulacral suckers are 

 evenly distributed and almost equally developed on all the tubes ; 

 between the five rows of ambulacral suckers are scattered irregu- 

 larly certain appendages resembling suckers, but found on exam- 

 ination not to be true locomotive suckers, and called on that 



account false ambulacra. These are the organs corresponding 

 to the warts around the mouth of the Synapta. Although the 

 ambulacral suckers are, as we have said, equally developed on all 

 the tubes, yet the Pentacta does not use them indiscriminately 

 as locomotive organs. In Pentacta, as well as in all Holothu- 

 rians, whether provided with ambulacral suckers, or, like the 

 Synapta and Caudina, deprived of them, the odd ambulacrum, 

 viz. the one placed opposite the madreporic body, is always used 

 to creep upon, and forms the under surface of the animal. 



The correspondence between the difierent phases of growth in 

 the young Pentacta, and the adult forms of the orders described 

 above, the Synapta, Caudina, Cuvieria, and Pentacta itself, is a 

 striking instance of the way in which embryonic forms illustrate 

 the relative standing of adult animals. In the earlier stages of 

 its development, the ambulacral tubes alone are developed in the 

 Pentacta ; in this condition it recalls the lower orders of Holo- 

 thurians, as the Synapta and Caudina ; then a sole is formed by 

 the greater development of three of the ambulacra, and in this 

 state it reminds us of the next in order, the Cuvieria, while it is 



Fig. 130. Pentacta frondosa ; expanded about one third the natural size. 



