Classification of Chordates: Protochordata 



405 



LATERAL TRUNK MUSCLES 



— SPINAL CORD 



NOTOCHORD 



DORSAL AORTA 



PRECARDINALV 

 -EPIBRANCHIAL GROOVE 



GILL LAMELLAE- 

 CARTILAGE BAR 

 GILL-RODS 



TRANSVERSE MUSCLE 

 I ""---VENTRAL AORTA 

 PERIBRANCHIAL CAVITY 



HYPOBRANCHIAL MUSCLE 

 METAPLEURAL FOLD 



A. AMPHIOXUS 



B AMMOCOETES. 



Fig. 316. Cross sections of (A.) Amphioxus and (B) ammocoetes (larval Pelro- 

 myzon) through the pharyngeal region, showing their fundamental resemblance. 

 (Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



Compared to a vertebrate, however, the cephalochordates lack 

 certain characteristic and highly important features. There are no 

 paired locomotor appendages of any sort. There is no definitely local- 

 ized blood-pumping organ such as the vertebrate heart. The blood is 

 colorless, containing no red blood-cells and few, if any, white cells 

 (leukocytes). In every known vertebrate, the conspicuous features of 

 the head are three pairs of highly specialized and functionally im- 

 portant sense-organs, the olfactory (nasal) organs, eyes, and ears. 

 Cephalochordates have no counterparts of these sense-organs. (Amphi- 

 oxus, however, is keenly sensitive to light.) In the absence of such 

 organs and possessing only a very feebly developed brain, there is no 

 need of a protective skeletal structure such as the "brain-case" or 

 cranium of vertebrates. Hence the cephalochordates, having no cra- 

 nium, are often called the Acrania. 



The cephalochordate embryo develops into a larva similar to that 

 of a tunicate, but the changes involved in the metamorphosis into the 

 adult are far less profound than those that occur in a tunicate. 



The Protochordata have received a large amount of careful study 

 because they seem to offer the possibility of the discovery of the long- 

 sought "connecting links" between vertebrates and invertebrates. The 



