No.1740. TYPES OF CRTNOID STEMS— CLARK. 213 



existence: this would seem to indicate a greal phylogenetic difference ; 

 but the species of Antedon are of exceedingly limited distribution; 

 those of Tropiomeira have a very wide distribution, necessitating a 

 prolonged free-swimming stage; arc we justified in saying thai the 

 larva 1 of Tropiometra may not turn out to be plutei or something like 

 them '. Echinoderm larvae 1 consider to be in exactly the same cate- 

 gory as arthropod larvae, useful in some ways, highly deceptive in 

 others. 



Attached by the central plate, our theoretical ancestral crinoid 

 has two possible courses to follow: (1) Tt may increase the area of 

 its attached base, or (2) it may increase its thickness, thus forming 

 a column. In recent forms the first possibility is realized in the young 

 of Holopus as figured by Air. Agassiz; the base has spread out enor- 

 mously, so that the animal presents a striking similarity to certain 

 low species of Balanus, the ten arms being countersunk, as it were, 

 in a depression at the apex of abroad low truncated cone. The sec- 

 ond possibility is exemplified among recent forms by the adult Holo- 

 pus; the base, instead of further spreading out, gradually becomes 

 elongated, so that the animal is raised up for a considerable distance 

 on a thick stalk. I can see no other explanation of the origin of the 

 base and the stalk in Holopus. 



Now, a stalk like that of Holopus is limited in its availability for 

 elongation; if it should grow to more than three or four times as 

 long as the minimum diameter, it would rapidly become exceedingly 

 brittle and liable to fracture by the contact of the animal with other 

 organisms, or even from the effects of wave motion. There are, 

 again, two possible lines of development: (1) The animal may break 

 off and thus secondarily become free, or (2) the column may break 

 in so far as the calcareous substance is concerned, yet remain in con- 

 tinuity through the organic base, thus developing an articulation 

 which would admit of a very considerable additional elongation, at 

 least double that of the original column. No recent crinoids are 

 known in which the first line of development obtains; but it is seen 

 in the fossil Edriocrinus. No crinoids are known in which the stem 

 is composed simply of two columnars, as would be the case in the 

 first stage of the second line of development. Rut suppose we carry 

 this line further; we have a crinoid attached by a stalk in which an 

 articulation has developed in the middle; such an articulation would 

 of necessity develop a fulcra! ridge running across the joint faces and 

 embracing the central canal, admitting of motion in a single plane, 

 perpendicular to that in which the original blow causing the fracture 

 was received. Stem growth would continue; but, as new deposi- 

 tion occurs only just under the calyx, only the outer columnar 

 would increase in length. Soon the outer columnar would become 

 so long as to become biittle, as did the original stem, and fracture 



