386 PHYSIOLOGY OF THE INVERTEBRATA. 



of these small forceps or grasping organs, that the animal is 

 capable of " climbing perpendicular or inclined surfaces of 

 rock, covered with waving sea-weeds." In the Adcridca and 

 the Holothuridca, the pedicellari^e are only rudimentary — 

 changed habits of life on the part of these animals have 

 caused the inherited appendages to dwindle from disuse. 

 For instance, " the Ophiuridea never climb sea- weed covered 

 rocks at all, and those starfishes which do so have their 

 ambulacral feet restricted to the ventral surface ; it would 

 therefore be useless for these animals to have well-developed 

 pedicellarifB, adapted to hold sea-weeds steady in the manner 

 which may be of so much use to the globular UcJmius, who 

 throws out on all sides feet feeling for attachments." 



Spatangus (one of the Echinided) craw^ls about somewhat 

 slower than Ecliinus ; and it is incapable of climbing per- 

 pendicular surfaces. When placed upon its back it has even 

 a greater difficulty in righting itself than Echinus. It rights 

 itself entirely by its long and mobile spines. 



(/) The Holotliuridea. — These animals " crawl slowly, and 

 indulge in prolonged periods of quiescence. They are, how- 

 ever, able to climb perpendicular surfaces." 



The Trichoscolices. 



The Turhcllaria. — Although the parenchyma of these 

 animals is contractile, they have only a very feebly-developed 

 muscular system. The muscular fibres appear to be un- 

 striated. The Tiirbellana are divided into («) the EhaMoccela 

 and (I)) the Dcndroccda. The smaller species of the first- 

 mentioned sub-order swim by means of their ciliated epithe- 

 lium ; whereas the larger species appear " to float from place 

 to place by means of their epithelium." The Dcndroccela, on 

 the other hand, crawl along somewhat in the manner of the 

 Gasteropoda. Sometimes the tentacle-like processes (situated 

 at the anterior end of the body) are used as oars when these 

 animals move upon the surface of the water. According to 



