Hemichordates and Lower Chordates

Phylum Hemichordata

Saccoglossus (Figure 1-1; p.3 & diagram below) - Be able to identify the following structures:
• proboscis
• collar
• trunk
• external gill slits

Phylum Chordata

Subphylum Urochordata
Molgula (larval stage; Figure 1-2, p.5) - Be able to identify the following structures:
• nerve cord
• notochord
• gill slits
• attachment disks
• incurrent opening
• excurrent opening
• atrium
• pharynx
Subphylum Cephalochordata - Amphioxus (Figure 1-3, p.7 & intestine Figure 1-4, p.9) - Be able to identify the following structures:
• notochord
• nerve cord
• pharynx
• pharyngeal bar
• pharyngeal slit
• oral cirri
• dorsal fin
• myomere
• myoseptum
• metapleural fold
• caudal fin
• atriopore
• anus

 

Subphylum Vertebrata

Class Agnatha
Order Petromyzontia - Lamprey (larval stage; Fig. 2-7, p. 26) - Be able to identify the following structures:
• notochord
• brain
• spinal cord
• pharynx
• gill pouch
• dorsal fin
• caudal fin
• oral tentacles
• intestine
• cloaca

Lampreys are considered a highly successful group of organisms. This is due in part to their freshwater larval stage (sedentary phase) that can last anywhere from 7-17 yrs. The ammocoete or larval lamprey resembles a small eel and lies concealed in silt deposits of rivers and streams. Suitable ammocoete habitat includes mud, silt or silt and sand and, varies in depth from a few inches to a foot or more. Larval lamprey habitat depends on stream gradients which determine velocity and the type of substrate particles that are deposited and accumulation of organic debris. In shallow, clear flowing water, the ammocoete positions its head such that its directed towards the current.

Ammocoetes are filter feeders, straining microorganisms from the water. The burrowing response of the larvae is its most characteristic behavioural pattern and it also serves to protect the ammocoete. Ammocoetes will burrow down tail first into the substrate such that the upper region of their bodies is vertical and the lower region is horizontal thereby forming a reverse L shape. Typical habitats include areas of eddies, backwater, inside of bends, behind obstructions where current velocity of the stream is below that of the main stream and, where organic material accumulates. Ammocoetes seldom leave their burrows but will change position frequently. Two important factors that will force an ammocoete to leave their burrows are low levels of oxygen and high levels of carbon dioxide.

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