400 THE CRANIATES AND THE ACRANIATES. 



to foreign objects by means of a sort of sucking or adhesive disc on the haemal side 

 of the head. Many parasitic copepods and cirripeds are permanently attached 

 in this manner, aided by a pair of modified appendages that have moved round 

 onto the haemal side of the head. In cirripeds the larva attaches itself, head 

 first, neural side down; it then turns a forward handspring on its rudimentary 

 adhesive antennae, bringing the neural side up; meantime an enormous out- 

 growth develops from the haemal surface of the head, forming the peduncle by 

 which the animal is permanently attached. (Fig. 274.) This extraordinary 

 mode of attachment, accompanied by the same peculiar rotation and cephalic 

 outgrowth, occurs with but slight variation in the tunicates, echinoderms, ptero- 

 branchia, phoronida, polyzoa, and brachiopods, in fact in every subdivision of 

 the acraniates except Amphioxus, the enteropneusta and chsetognatha. 



Mantle. — Before the young cirriped becomes attached, the valves of the 

 thoracic shield make their appearance as a longitudinal circular fold. The free 

 edge of the fold gradually extends toward the neural surface, enclosing the body 

 and appendages in a large vestibular, atrial, or branchial chamber. (Figs. 289, 

 281.) A similar mantle forms a familiar and conspicuous feature in the larval 

 and adult stages of the tunicates (Figs. 284-286), echinoderms (Figs. 291, 

 295), polyzoa (Fig. 301), phoronida (Fig. 305), and brachiopods (Fig. 304). In 

 the polyzoa and echinoderms, the vestibule may develop very early as a closed 

 chamber; but it is soon ruptured by the growing appendages within, which then 

 protrude through the opening in the same manner as those of a cirriped. In 

 the enteropneusta the mantle consists of two longitudinal pleural folds that form 

 an imperfect branchial chamber. (Fig. 298.) 



The rudimentary mantle fold is a conspicuous feature of the larvas of cirripeds, 

 echinoderms, enteropneusta, polyzoa, phoronida, and brachiopods. Its free 

 margin may be drawn out into characteristic projections or lobes, that, heavily 

 ciliated, form the primary longitudinal ciliated band characteristic of the naupula. 

 It should not be confused with the transverse ciliated band characteristic of the 

 trochophore larva. (Figs. 267-296.) 



Skeleton. — An endocranium occurs only in the enteropneusta and chsto- 

 gnatha. In the enteropneusta it consists of a low grade fibro-cartilage, probably 

 of mesodermic origin, and comparable with the primitive endocranium of the 

 phyllopods. See page 312. No neural or branchial cartilages appear, but 

 in Amphioxus and the enteropneusta, a complicated framework of chitenoid gill 

 bars supports the margins of the gill clefts and the tongue bars. 



The exoskeleton may consist of a voluminous chitenoid, celluloid, or gelatin- 

 oid secretion of the ectoderm; it is not shed at regular intervals, as in the arthropods, 

 but is retained throughout life. The only exception appears to be the appendicu- 

 laria, which often shed their enormous " gelatinous house " soon after its formation. 

 It may be heavily calcified, forming characteristic polygonal plates, and greatly 

 complicated by epidermal folds and channels containing vascular or other tissues 

 (cirripeds, tunicates, brachiopods). 



