CAMBRIAN INVERTEBRATES 



129 



CH/ETOGNATHA 



overlapping groups of scale-like dorsal spines comparable only 

 to those of the living Aphroditidae. An example of this latter 

 family is Polynoe sguamala, furnished with dorsal scales. Still 

 other recent forms, such as Palmyra aiirij'era Savigny, have 

 groups of spinous scales closely 

 resembling those of Canadia. 



Even the modern freely pro- 

 pelled Chcrtognatha have their 

 representatives in the mid- 

 Cambrian, for to no other group 

 of invertebrates can Amlskwia 

 sagittiformis Walcott (Fig. 26) 

 be referred, so far as we can 

 judge by its external form. As 

 in the recent Sagitta the body 

 is divided into head, trunk, and 

 a somewhat fish-like tail. Its 

 single pair of fins of chaetognath 

 type would perhaps give a 

 clearer aflfinity to the genus 

 Spadella. The conspicuous pair 

 of tentacles which surmounts 

 the head is absent in modern 



chaetognaths, although some recent species show a pair of sen- 

 sory papillae mounted on a stalk on either side of the head, as 

 in Spadella cephaloptera Bush. The digestive canal and other 

 digestive organs appear through the thin walls of the body. 



A modern group of jellyfishes, the Scyphomedusa? (Fig. 27), 

 is represented by the Middle Cambrian Peyioia nathorsti, the 

 elliptical disk of which is seen from below. Although this 

 fossil species is ascribed by Walcott to the group Rhizostomae 

 because of a lack of marginal tentacles, the thirty-two radiat- 



FiG. 26. Freely Swimming Ch^tog- 

 NATHS, Cambrian and Recent. 



Amishcia sagittiformis, a mid-Cambrian 

 form (after Walcott), has a body di- 

 vided into head, trunk, and tail like the 

 recent Sagitta, as seen in S. gardincri. 



