453 THE LANCELETS— CEPHALOCHORDA 



External Appearance. — Lancelets are semi-transparent, often 

 slightly iridescent animals, pointed at both ends, two or three 

 inches long, with the body slightly flattened from side to side. 

 The living animals are much plumper than those which have 

 been preserved in spirit would suggest. All along the body 

 there are V-shaped markings, with the apex of the V directed 

 forwards ; these are due to cross-partitions of connective tissue 

 that divide the longitudinal muscles of the sides of the body 

 into segments or successive blocks. An interesting detail is 

 that the segments of the two sides are alternate, not opposite. 

 Thus the 27th block (or myotome) on the left is opposite the 

 26th partition (or myocomma) on the right. 



The mouth is just behind the anterior tip of the body and 

 is surrounded by delicate ciliated processes, called cirri, which 

 waft in food particles. The anus opens ventrally on the left 

 side not far from the posterior end. A ventral groove extends 

 from the mouth backwards for about two-thirds of the length 

 of the body, and is bounded by hollow side folds (metapleural 

 folds), which converge at an opening called the atrial pore. A 

 delicate fold of skin, supported by minute rodlets, runs along 

 the middle line of the back, round the tail end, and along the 

 ventral surface as far as the atriopore. 



It is not the aim of this book to give an account of the de- 

 tailed structure of types, and we do not propose to depart from 

 this rule even in the case of the lancelets which stand so much 

 by themselves. There are many admirable anatomical ac- 

 counts {e.g., those of Lankester, Sedgwick, and Herdman) and 

 the annotated diagrams on Plate XXXIV. will give a clear 

 picture of the general features — which is all that we require 

 for our present purpose. Let us seek, however, to sum up 

 these general features so as to make plain that the Lancelets 

 are indeed very remarkable animals. 



The Lancelets are chordate animals inasmuch as they 

 possess (1) a dorsal tubular nerve-cord (without more than a 

 hint of a brain) ; (2) a dorsal supporting axis (a persistent un- 

 segmented notochord) ; and (3) a number of gill-slits, which 

 open in the young animal from the pharynx to the exterior. 

 In regard to each of these three points, however, there are remark- 

 able peculiarities : — 



(1) The most anterior part of the nervous system is not 



