IJIO MR. E. R. LANKESTER OX SOME LOWER ANNELIDS. 



are one after the other concerned in their production, and may throw some additional 

 light on the suhject mentioned in the last paragraph. Professor Claus, in the * TTiirz- 

 hur v 7. itsehrii't,' has a short paper on the order of the growth of zooids in a chain. 

 lie noted that as to age or condition of development they do not follow as they are 

 attached, one behind the other, the first, second, third, and fourth being relatively 

 first. 9< <nd, third, and fourth in Bee, but that, whilst the first is of course the oldest 



b 



or | rent zooid, the second in position is one of the newest, the second in age bein 

 n ar t lie middle of the chain. ll<' lias observed chains of as many as sixteen zooids, and 

 gives the follow ing formula as < \pr sing the simple law of their relative age or period 

 of development. 1 1 n be the number of zooids in a chain, the formula gives the sequence 

 of looids in aga, each number representing the zooid whose position is indicated by that 



number 



_ n , n , , 8fi . t n . , 5» . ., Sn -. 7n . -. 

 *• 2 + 1, 4 +1 ' T + 1, 8 +1, "8 +1 * ¥ +1 ' ¥ + 1 



__, 9 n 5n 3m In n Sn n 



This ohviou s 1 y results from the fact that each zooid is continually growing and producing 

 tr A\ zooids, and that, even before a zooid has become marked off from its parent in 

 front, the parent has begun to produce a new zooid. That this is the case can be 

 eatisnietorily determined by examining chains of zooids, whose relative ages are at once 

 letennined h\ their eo.uparative development. The simplest Chcetogaster I have seen 

 is one consisting of a head and six segments (indicated by the abdominal bristle-bundles), 

 between the third and fourth of which a fresh segment was beginning to make its 



ippearan \ From the examination of various specimens, for one cannot be kept and 



ace.l, the process appears to continue thus :— An eighth segment also begins to grow 



simultaneously with the seventh, growing between the third and fourth of the original 



worm ; and then a homogeneous blastema is deposited about and between the new fourth 



nd old fourth segments ; in this, by anterior growth, the head of a zooicl whose body is 

 formed b the old fourth, fifth, and sixth segments is developed, and, simultaneously, 

 a n w segment is added to the new fourth segment, and the terminal segment of the 



chain. ! fore this has proceeded long, new segments arise in another plane of growth, 



namely, one 1 tween the third and fourth of the first zooid, and one between the third 



and fourth of the new zooid; and thus the whole process repeats itself in each case, 

 both m i urent and zooid. The three segments succeeding a head are never separated, 



th remain permanently attached when once grown; but the fourth segment, with the 



i that develope behind it, is continually being detached by the growth of a head 



tween it and it. parent The whole phenomenon is then reduced to this simple 



statvmen :-An iiuhvulual Ckatogaster consisting of a head and three segments is 



continually producing additional sermon to i^ ™ + ■ n „ 



41 .. ' l . * uul0Ilai segments, by posterior growth, from its third segment ; 



the fourth segment, count if from Hia 1^0,1 i • 1? , 



tudina) eohe*», ' th. ,l, : ■ " Ue ' ,j ' V- !"* the iutesration ° r long1 ' 



^ the I „ acy of t^Z^ZlZ J ""IS?* o-rpowered at the faott. 



se ment-zooKl to anterior growth, or to its completion by the 



