Figure 6.

(A) Illustration of a cydippid ctenophore, showing the oral and aboral poles bearing the mouth (m) and statocyst (sc), respectively. Ctenophores possess eight comb rows (cr), each made up of a series of comb plates (cp) which beat in the oral-aboral direction during forward swimming, or aboral-oral direction during reverse and rotational swimming. Geotactic control of comb row beating occurs via signal transduction from the statocyst, a ciliated gravitometric organ, to the beginning of each comb row via ciliated grooves (cg). Tentacles (t) and tentilla (tl) bear colloblasts, laden with adhesive granules used for prey capture; injested food enters the mouth into the pharynx (p), and eventually the stomach (s) and digestive system. Inset: Side view of two balancers (b) of the statocyst of an animal in the horizontal position, connected at their tips to the statolith (sl). Weight from the statolith mechanically deflects the balancers either toward or away from the midline (m), mechanically activating the beating of balancer cilia; these then activate waves of beating in the ciliated grooves (cg) which propagate to the comb rows. (B) Illustration of the pre-synaptic triad of ctenophore synapses, consisting of rows of synaptic vesicles (sv) arranged along the membrane, adjacent to a finger-like projection of smooth endoplasmic reticulum, which lacks ribosomes (r) of the rough endoplasmic reticulum, and one or several large mitochondria (mi). n, nucleus; g, Golgi, c.v, cytoplasmic vesicles; co, post-synaptic dense coat; p, pre-synaptic dense projections. Reprinted with permission from Hernandez-Nicaise (1973a).